/* ============================================
   DESIGN TOKENS — Soft, Eye-Friendly Light Theme
   ============================================ */
:root {
    /* Warm, soft background palette */
    --clr-bg: #f8f6f3;
    --clr-bg-warm: #faf8f5;
    --clr-bg-alt: #f1eeea;
    --clr-surface: #ffffff;
    --clr-surface-2: #f3f1ee;
    --clr-surface-3: #eae7e2;
    --clr-border: rgba(0, 0, 0, 0.06);
    --clr-border-hover: rgba(0, 0, 0, 0.12);

    /* Text — warm grays, never pure black */
    --clr-text: #2d2a26;
    --clr-text-secondary: #5a5650;
    --clr-text-muted: #8a8580;
    --clr-text-dim: #b0aaa4;

    /* Soft accent — warm indigo / lavender */
    --clr-accent: #6366f1;
    --clr-accent-light: #818cf8;
    --clr-accent-soft: rgba(99, 102, 241, 0.08);
    --clr-accent-softer: rgba(99, 102, 241, 0.04);
    --gradient-primary: linear-gradient(135deg, #6366f1, #8b5cf6);
    --gradient-soft: linear-gradient(135deg, #6366f1, #a78bfa);

    /* Success / Error — muted, soft */
    --clr-success: #16a34a;
    --clr-success-bg: #f0fdf4;
    --clr-success-border: #bbf7d0;
    --clr-error: #dc2626;
    --clr-error-bg: #fef2f2;
    --clr-error-border: #fecaca;

    /* Spacing scale */
    --space-xs: 0.25rem;
    --space-sm: 0.5rem;
    --space-md: 1rem;
    --space-lg: 1.5rem;
    --space-xl: 2rem;
    --space-2xl: 3rem;
    --space-3xl: 3.5rem;
    --space-4xl: 4.5rem;

    /* Radius */
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 16px;
    --radius-xl: 24px;
    --radius-2xl: 32px;
    --radius-full: 9999px;

    /* Typography */
    --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
    --font-mono: 'JetBrains Mono', 'Fira Code', monospace;

    /* Soft shadows */
    --shadow-xs: 0 1px 2px rgba(0, 0, 0, 0.04);
    --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.06), 0 1px 2px rgba(0, 0, 0, 0.04);
    --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.06), 0 2px 4px rgba(0, 0, 0, 0.04);
    --shadow-lg: 0 8px 30px rgba(0, 0, 0, 0.07), 0 4px 8px rgba(0, 0, 0, 0.04);
    --shadow-xl: 0 16px 50px rgba(0, 0, 0, 0.08);
    --shadow-card: 0 2px 8px rgba(0, 0, 0, 0.04), 0 0 0 1px rgba(0, 0, 0, 0.03);
    --shadow-card-hover: 0 8px 24px rgba(0, 0, 0, 0.08), 0 0 0 1px rgba(0, 0, 0, 0.05);

    /* Transitions */
    --ease-out: cubic-bezier(0.16, 1, 0.3, 1);
    --ease-spring: cubic-bezier(0.34, 1.56, 0.64, 1);
    --dur-fast: 0.15s;
    --dur-md: 0.25s;
    --dur-slow: 0.4s;

    /* Header bg for backdrop */
    --header-bg: rgba(248, 246, 243, 0.85);
    --nav-mobile-bg: rgba(248, 246, 243, 0.97);
}

/* ============================================
   DARK MODE THEME
   ============================================ */
[data-theme="dark"] {
    --clr-bg: #0f1117;
    --clr-bg-warm: #151820;
    --clr-bg-alt: #1a1d28;
    --clr-surface: #1e2130;
    --clr-surface-2: #252839;
    --clr-surface-3: #2e3245;
    --clr-border: rgba(255, 255, 255, 0.07);
    --clr-border-hover: rgba(255, 255, 255, 0.14);

    --clr-text: #e8e6e3;
    --clr-text-secondary: #a8a4a0;
    --clr-text-muted: #757170;
    --clr-text-dim: #555250;

    --clr-accent: #818cf8;
    --clr-accent-light: #a5b4fc;
    --clr-accent-soft: rgba(129, 140, 248, 0.12);
    --clr-accent-softer: rgba(129, 140, 248, 0.06);
    --gradient-primary: linear-gradient(135deg, #818cf8, #a78bfa);
    --gradient-soft: linear-gradient(135deg, #818cf8, #c4b5fd);

    --clr-success: #4ade80;
    --clr-success-bg: rgba(22, 163, 74, 0.1);
    --clr-success-border: rgba(74, 222, 128, 0.2);
    --clr-error: #f87171;
    --clr-error-bg: rgba(220, 38, 38, 0.1);
    --clr-error-border: rgba(248, 113, 113, 0.2);

    --shadow-xs: 0 1px 2px rgba(0, 0, 0, 0.2);
    --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.3), 0 1px 2px rgba(0, 0, 0, 0.2);
    --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.3), 0 2px 4px rgba(0, 0, 0, 0.2);
    --shadow-lg: 0 8px 30px rgba(0, 0, 0, 0.35), 0 4px 8px rgba(0, 0, 0, 0.2);
    --shadow-xl: 0 16px 50px rgba(0, 0, 0, 0.4);
    --shadow-card: 0 2px 8px rgba(0, 0, 0, 0.2), 0 0 0 1px rgba(255, 255, 255, 0.04);
    --shadow-card-hover: 0 8px 24px rgba(0, 0, 0, 0.3), 0 0 0 1px rgba(255, 255, 255, 0.06);

    --header-bg: rgba(15, 17, 23, 0.88);
    --nav-mobile-bg: rgba(15, 17, 23, 0.97);
}


/* ============================================
   RESET & BASE
   ============================================ */
*, *::before, *::after {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
    scroll-padding-top: 80px;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
    font-size: 16px;
}

body {
    font-family: var(--font-sans);
    background: var(--clr-bg);
    color: var(--clr-text);
    line-height: 1.65;
    overflow-x: hidden;
    min-height: 100vh;
}

a {
    color: inherit;
    text-decoration: none;
}

button {
    cursor: pointer;
    font-family: inherit;
    border: none;
    background: none;
    color: inherit;
}

input {
    font-family: inherit;
    border: none;
    outline: none;
    background: none;
    color: inherit;
}

img {
    max-width: 100%;
    display: block;
}

kbd {
    font-family: var(--font-mono);
    background: var(--clr-surface-2);
    padding: 0.15em 0.5em;
    border-radius: 6px;
    font-size: 0.85em;
    border: 1px solid var(--clr-border);
    color: var(--clr-accent);
    font-weight: 500;
}


/* ============================================
   INTERACTIVE MATH SYMBOLS BACKGROUND (Canvas)
   ============================================ */
.bg-math-canvas {
    position: fixed;
    inset: 0;
    z-index: -2;
    pointer-events: none;
    width: 100%;
    height: 100%;
}

.bg-math {
    position: fixed;
    inset: 0;
    z-index: -1;
    overflow: hidden;
    pointer-events: none;
}

/* Subtle lavender gradient accents in corners */
.bg-math::before {
    content: '';
    position: absolute;
    inset: 0;
    background:
        radial-gradient(ellipse 55% 45% at 0% 0%, rgba(139, 92, 246, 0.07), transparent 60%),
        radial-gradient(ellipse 45% 55% at 100% 100%, rgba(139, 92, 246, 0.05), transparent 60%),
        radial-gradient(ellipse 35% 35% at 100% 0%, rgba(99, 102, 241, 0.04), transparent 50%),
        radial-gradient(ellipse 35% 35% at 0% 100%, rgba(236, 72, 153, 0.03), transparent 50%);
    z-index: 0;
}

[data-theme="dark"] .bg-math::before {
    background:
        radial-gradient(ellipse 55% 45% at 0% 0%, rgba(139, 92, 246, 0.12), transparent 60%),
        radial-gradient(ellipse 45% 55% at 100% 100%, rgba(139, 92, 246, 0.09), transparent 60%),
        radial-gradient(ellipse 35% 35% at 100% 0%, rgba(99, 102, 241, 0.07), transparent 50%),
        radial-gradient(ellipse 35% 35% at 0% 100%, rgba(236, 72, 153, 0.05), transparent 50%);
}


/* ============================================
   HEADER
   ============================================ */
.header {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    z-index: 100;
    padding: var(--space-md) var(--space-xl);
    backdrop-filter: blur(16px);
    -webkit-backdrop-filter: blur(16px);
    background: var(--header-bg);
    border-bottom: 1px solid var(--clr-border);
    transition: all var(--dur-md) var(--ease-out);
}

.header-inner {
    max-width: 1100px;
    margin: 0 auto;
    display: flex;
    align-items: center;
    justify-content: space-between;
}

.logo {
    display: flex;
    align-items: center;
    gap: var(--space-sm);
    font-weight: 700;
    font-size: 1.1rem;
    letter-spacing: -0.02em;
    color: var(--clr-text);
    transition: opacity var(--dur-fast);
}

.logo:hover {
    opacity: 0.7;
}

.logo-icon {
    font-size: 1.3rem;
}

.nav {
    display: flex;
    gap: 2px;
}

.nav-link {
    padding: var(--space-sm) var(--space-md);
    border-radius: var(--radius-sm);
    font-size: 0.875rem;
    font-weight: 500;
    color: var(--clr-text-muted);
    transition: all var(--dur-fast);
    position: relative;
}

.nav-link:hover {
    color: var(--clr-text);
    background: var(--clr-accent-softer);
}

.nav-link.active {
    color: var(--clr-accent);
    background: var(--clr-accent-soft);
}

/* Theme toggle button */
.theme-toggle {
    display: flex;
    align-items: center;
    justify-content: center;
    width: 40px;
    height: 40px;
    border-radius: var(--radius-full);
    background: var(--clr-surface-2);
    border: 1px solid var(--clr-border);
    color: var(--clr-text-muted);
    transition: all var(--dur-md) var(--ease-out);
    cursor: pointer;
    flex-shrink: 0;
    margin-left: var(--space-sm);
    position: relative;
}

.theme-toggle:hover {
    background: var(--clr-accent-soft);
    color: var(--clr-accent);
    border-color: var(--clr-accent-light);
    transform: rotate(15deg);
}

.theme-icon {
    position: absolute;
    transition: opacity var(--dur-md) var(--ease-out), transform var(--dur-md) var(--ease-out);
}

/* Light mode: show moon (indicates "switch to dark"), hide sun */
.sun-icon {
    opacity: 0;
    transform: rotate(90deg) scale(0.5);
}

.moon-icon {
    opacity: 1;
    transform: rotate(0deg) scale(1);
}

/* Dark mode: show sun (indicates "switch to light"), hide moon */
[data-theme="dark"] .sun-icon {
    opacity: 1;
    transform: rotate(0deg) scale(1);
}

[data-theme="dark"] .moon-icon {
    opacity: 0;
    transform: rotate(-90deg) scale(0.5);
}

/* Mobile menu button */
.mobile-menu-btn {
    display: none;
    flex-direction: column;
    gap: 5px;
    padding: var(--space-sm);
}

.mobile-menu-btn span {
    display: block;
    width: 22px;
    height: 2px;
    background: var(--clr-text);
    border-radius: 2px;
    transition: all var(--dur-md) var(--ease-out);
}

.mobile-menu-btn.active span:nth-child(1) {
    transform: rotate(45deg) translate(5px, 5px);
}

.mobile-menu-btn.active span:nth-child(2) {
    opacity: 0;
}

.mobile-menu-btn.active span:nth-child(3) {
    transform: rotate(-45deg) translate(5px, -5px);
}


/* ============================================
   HERO
   ============================================ */
.hero {
    min-height: 90vh;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 120px var(--space-xl) var(--space-xl);
    gap: var(--space-4xl);
    max-width: 1100px;
    margin: 0 auto;
}

.hero-content {
    flex: 1;
    max-width: 520px;
}

.hero-badge {
    display: inline-flex;
    align-items: center;
    gap: var(--space-sm);
    padding: 6px var(--space-md);
    background: var(--clr-surface);
    border: 1px solid var(--clr-border);
    border-radius: var(--radius-full);
    font-size: 0.8rem;
    font-weight: 600;
    color: var(--clr-text-muted);
    margin-bottom: var(--space-xl);
    box-shadow: var(--shadow-xs);
    animation: fadeInUp 0.6s var(--ease-out) both;
}

.badge-dot {
    width: 7px;
    height: 7px;
    border-radius: 50%;
    background: var(--clr-success);
    animation: pulse 2.5s ease-in-out infinite;
}

@keyframes pulse {
    0%, 100% { opacity: 1; transform: scale(1); }
    50%      { opacity: 0.5; transform: scale(0.85); }
}

.hero-title {
    font-size: clamp(2.5rem, 5.5vw, 4rem);
    font-weight: 800;
    line-height: 1.1;
    letter-spacing: -0.03em;
    margin-bottom: var(--space-lg);
    color: var(--clr-text);
    animation: fadeInUp 0.6s var(--ease-out) 0.1s both;
}

.gradient-text {
    background: var(--gradient-soft);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
}

.hero-subtitle {
    font-size: 1.1rem;
    color: var(--clr-text-secondary);
    line-height: 1.75;
    margin-bottom: var(--space-2xl);
    animation: fadeInUp 0.6s var(--ease-out) 0.2s both;
}

.hero-cta {
    display: inline-flex;
    align-items: center;
    gap: var(--space-sm);
    padding: 14px var(--space-xl);
    background: var(--gradient-primary);
    border-radius: var(--radius-full);
    font-weight: 600;
    font-size: 0.95rem;
    color: #fff;
    transition: all var(--dur-md) var(--ease-out);
    box-shadow: 0 4px 16px rgba(99, 102, 241, 0.25);
    animation: fadeInUp 0.6s var(--ease-out) 0.3s both;
    margin-bottom: var(--space-2xl);
}

.hero-cta:hover {
    transform: translateY(-2px);
    box-shadow: 0 8px 24px rgba(99, 102, 241, 0.35);
}

.hero-cta svg {
    transition: transform var(--dur-md) var(--ease-out);
}

.hero-cta:hover svg {
    transform: translateX(4px);
}

/* Hero visual — animated formula */
.hero-visual {
    flex-shrink: 0;
    animation: fadeInUp 0.8s var(--ease-out) 0.4s both;
}

.formula-display {
    display: flex;
    align-items: center;
    gap: var(--space-2xl);
    padding: var(--space-3xl) var(--space-3xl);
    background: var(--clr-surface);
    border: 1px solid var(--clr-border);
    border-radius: var(--radius-2xl);
    box-shadow: var(--shadow-lg);
    animation: gentleFloat 7s ease-in-out infinite;
}

@keyframes gentleFloat {
    0%, 100% { transform: translateY(0); }
    50%      { transform: translateY(-8px); }
}

.formula-fraction {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: var(--space-sm);
}

.formula-num,
.formula-den {
    font-family: var(--font-mono);
    font-size: 2.25rem;
    font-weight: 700;
    color: var(--clr-accent);
}

.formula-line {
    width: 55px;
    height: 3px;
    background: var(--gradient-primary);
    border-radius: var(--radius-full);
    opacity: 0.6;
}

.formula-equals {
    font-size: 2.25rem;
    font-weight: 300;
    color: var(--clr-text-dim);
}

/* Interactive hero formula animation states */
.formula-num,
.formula-den {
    transition: all 0.4s var(--ease-out);
    min-width: 40px;
    text-align: center;
    display: inline-block;
}

.formula-num.formula-unknown,
.formula-den.formula-unknown {
    color: var(--clr-accent-light);
    animation: unknownPulse 1.5s ease-in-out infinite;
    text-shadow: 0 0 20px rgba(99, 102, 241, 0.3);
}

.formula-num.formula-solving,
.formula-den.formula-solving {
    animation: solvePop 0.5s var(--ease-spring) both;
    color: var(--clr-success);
    text-shadow: 0 0 16px rgba(22, 163, 74, 0.25);
}

.formula-num.formula-fadeout,
.formula-den.formula-fadeout {
    opacity: 0;
    transform: translateY(-10px);
    transition: all 0.3s var(--ease-out);
}

.formula-num.formula-fadein,
.formula-den.formula-fadein {
    animation: fadeInValue 0.4s var(--ease-spring) both;
}

@keyframes unknownPulse {
    0%, 100% { opacity: 1; transform: scale(1); }
    50%      { opacity: 0.6; transform: scale(1.08); }
}

@keyframes solvePop {
    0%   { opacity: 0; transform: scale(0.5); }
    60%  { transform: scale(1.15); }
    100% { opacity: 1; transform: scale(1); }
}

@keyframes fadeInValue {
    0%   { opacity: 0; transform: translateY(10px); }
    100% { opacity: 1; transform: translateY(0); }
}

/* Hero formula label */
.hero-formula-label {
    text-align: center;
    margin-top: var(--space-md);
    font-size: 0.82rem;
    font-weight: 600;
    color: var(--clr-text-muted);
    letter-spacing: 0.02em;
    transition: all 0.3s var(--ease-out);
    min-height: 1.4em;
}

.hero-formula-label.label-solved {
    color: var(--clr-success);
}


/* ============================================
   SECTIONS — SHARED
   ============================================ */
section {
    padding: var(--space-2xl) var(--space-xl);
}

.section-header {
    text-align: center;
    max-width: 560px;
    margin: 0 auto var(--space-2xl);
}

.section-title {
    font-size: clamp(1.6rem, 3vw, 2.25rem);
    font-weight: 800;
    letter-spacing: -0.02em;
    margin-bottom: var(--space-sm);
    color: var(--clr-text);
}

.section-desc {
    font-size: 1rem;
    color: var(--clr-text-muted);
    line-height: 1.7;
}

.section-desc strong {
    color: var(--clr-text-secondary);
    font-weight: 600;
}


/* ============================================
   CALCULATOR CARD
   ============================================ */
.calculator-section {
    max-width: 1100px;
    margin: 0 auto;
}

.calculator-card {
    max-width: 660px;
    margin: 0 auto;
    background: var(--clr-surface);
    border: 1px solid var(--clr-border);
    border-radius: var(--radius-2xl);
    padding: var(--space-2xl) var(--space-2xl) var(--space-xl);
    box-shadow: var(--shadow-lg);
    transition: box-shadow var(--dur-md);
}

.calculator-card:hover {
    box-shadow: var(--shadow-xl);
}

/* Instruction */
.calc-instruction {
    display: flex;
    align-items: center;
    gap: 10px;
    padding: var(--space-md) var(--space-lg);
    background: var(--clr-accent-soft);
    border-radius: var(--radius-md);
    font-size: 0.875rem;
    color: var(--clr-text-secondary);
    margin-bottom: var(--space-2xl);
    border: 1px solid rgba(99, 102, 241, 0.1);
}

.calc-instruction svg {
    flex-shrink: 0;
    color: var(--clr-accent);
}

/* Proportion inputs grid */
.proportion-grid {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: var(--space-xl);
    margin-bottom: var(--space-2xl);
}

.fraction-group {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 10px;
    flex: 1;
    max-width: 210px;
}

.input-wrapper {
    width: 100%;
    position: relative;
}

.input-label {
    position: absolute;
    top: -9px;
    left: 14px;
    font-size: 0.7rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.1em;
    color: var(--clr-text-dim);
    background: var(--clr-surface);
    padding: 0 6px;
    z-index: 1;
    transition: color var(--dur-fast);
}

.calc-input {
    width: 100%;
    padding: 16px var(--space-lg);
    background: var(--clr-bg-warm);
    border: 2px solid var(--clr-surface-3);
    border-radius: var(--radius-md);
    font-size: 1.35rem;
    font-family: var(--font-mono);
    font-weight: 600;
    text-align: center;
    color: var(--clr-text);
    transition: all var(--dur-md) var(--ease-out);
}

.calc-input::placeholder {
    color: var(--clr-text-dim);
    font-weight: 400;
}

.calc-input:focus {
    border-color: var(--clr-accent-light);
    background: var(--clr-surface);
    box-shadow: 0 0 0 4px var(--clr-accent-soft);
}

.input-wrapper:focus-within .input-label {
    color: var(--clr-accent);
}

.calc-input.solved {
    border-color: var(--clr-success);
    background: var(--clr-success-bg);
    box-shadow: 0 0 0 4px rgba(22, 163, 74, 0.08);
}

.calc-input:disabled {
    opacity: 0.5;
    cursor: not-allowed;
    background: var(--clr-surface-3);
    border-color: var(--clr-border);
}

.calc-input.error {
    border-color: var(--clr-error);
    background: var(--clr-error-bg);
    box-shadow: 0 0 0 4px rgba(220, 38, 38, 0.06);
}

.fraction-divider {
    width: 70%;
    height: 3px;
    background: var(--clr-surface-3);
    border-radius: var(--radius-full);
}

.equals-sign {
    font-size: 2rem;
    font-weight: 300;
    color: var(--clr-text-dim);
    flex-shrink: 0;
}

/* Buttons */
.calc-actions {
    display: flex;
    gap: var(--space-md);
    justify-content: center;
    margin-bottom: var(--space-lg);
}

.btn {
    display: inline-flex;
    align-items: center;
    gap: var(--space-sm);
    padding: 13px 28px;
    border-radius: var(--radius-full);
    font-size: 0.95rem;
    font-weight: 600;
    transition: all var(--dur-md) var(--ease-out);
}

.btn-primary {
    background: var(--gradient-primary);
    color: #fff;
    box-shadow: 0 4px 14px rgba(99, 102, 241, 0.25);
}

.btn-primary:hover {
    transform: translateY(-2px);
    box-shadow: 0 6px 20px rgba(99, 102, 241, 0.35);
}

.btn-primary:active {
    transform: translateY(0) scale(0.98);
}

.btn-secondary {
    background: var(--clr-surface-2);
    border: 1px solid var(--clr-border);
    color: var(--clr-text-muted);
}

.btn-secondary:hover {
    background: var(--clr-surface-3);
    color: var(--clr-text);
}

/* Result */
.result-panel {
    padding: var(--space-xl);
    background: var(--clr-success-bg);
    border: 1px solid var(--clr-success-border);
    border-radius: var(--radius-lg);
    animation: slideUp 0.4s var(--ease-spring) both;
}

.result-header {
    display: flex;
    align-items: center;
    gap: var(--space-sm);
    font-weight: 700;
    color: var(--clr-success);
    margin-bottom: var(--space-md);
    font-size: 0.85rem;
    text-transform: uppercase;
    letter-spacing: 0.08em;
}

.result-value {
    font-family: var(--font-mono);
    font-size: 1.75rem;
    font-weight: 700;
    color: var(--clr-text);
    margin-bottom: var(--space-md);
}

.result-equation {
    font-family: var(--font-mono);
    font-size: 1rem;
    color: var(--clr-text-secondary);
    padding: var(--space-md);
    background: rgba(22, 163, 74, 0.06);
    border-radius: var(--radius-sm);
    margin-bottom: var(--space-md);
    border: 1px solid rgba(22, 163, 74, 0.1);
}

.result-steps {
    font-size: 0.875rem;
    color: var(--clr-text-muted);
    line-height: 1.85;
}

.result-steps strong {
    color: var(--clr-text);
}

/* Error */
.error-panel {
    display: flex;
    align-items: center;
    gap: 10px;
    padding: var(--space-lg);
    background: var(--clr-error-bg);
    border: 1px solid var(--clr-error-border);
    border-radius: var(--radius-lg);
    color: var(--clr-error);
    font-weight: 500;
    font-size: 0.925rem;
    animation: shake 0.35s var(--ease-out);
}

.error-panel svg {
    flex-shrink: 0;
}

@keyframes shake {
    0%, 100% { transform: translateX(0); }
    20%      { transform: translateX(-5px); }
    40%      { transform: translateX(5px); }
    60%      { transform: translateX(-3px); }
    80%      { transform: translateX(3px); }
}

.hidden {
    display: none !important;
}


/* ============================================
   HOW IT WORKS
   ============================================ */
.how-section {
    max-width: 1100px;
    margin: 0 auto;
}

.steps-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: var(--space-lg);
}

.step-card {
    padding: var(--space-2xl) var(--space-xl);
    background: var(--clr-surface);
    border: 1px solid var(--clr-border);
    border-radius: var(--radius-xl);
    text-align: center;
    box-shadow: var(--shadow-card);
    transition: all var(--dur-md) var(--ease-out);
}

.step-card:hover {
    transform: translateY(-4px);
    box-shadow: var(--shadow-card-hover);
}

.step-number {
    width: 48px;
    height: 48px;
    display: flex;
    align-items: center;
    justify-content: center;
    margin: 0 auto var(--space-lg);
    font-family: var(--font-mono);
    font-size: 1.15rem;
    font-weight: 700;
    background: var(--clr-accent-soft);
    color: var(--clr-accent);
    border-radius: var(--radius-md);
    border: 1px solid rgba(99, 102, 241, 0.12);
}

.step-title {
    font-size: 1.1rem;
    font-weight: 700;
    margin-bottom: var(--space-sm);
    color: var(--clr-text);
}

.step-desc {
    font-size: 0.9rem;
    color: var(--clr-text-muted);
    line-height: 1.7;
}

.step-desc strong {
    color: var(--clr-text-secondary);
    font-weight: 600;
}


/* ============================================
   EXAMPLES
   ============================================ */
.examples-section {
    max-width: 1100px;
    margin: 0 auto;
}

.examples-grid {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: var(--space-md);
    max-width: 720px;
    margin: 0 auto;
}

.example-card {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: var(--space-md);
    padding: var(--space-xl) var(--space-md);
    background: var(--clr-surface);
    border: 1px solid var(--clr-border);
    border-radius: var(--radius-lg);
    box-shadow: var(--shadow-card);
    transition: all var(--dur-md) var(--ease-out);
    cursor: pointer;
}

.example-card:hover {
    border-color: var(--clr-accent-light);
    transform: translateY(-3px);
    box-shadow: var(--shadow-card-hover);
    background: var(--clr-accent-softer);
}

.example-fraction {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 2px;
    font-family: var(--font-mono);
    font-size: 0.95rem;
    font-weight: 600;
    color: var(--clr-text-secondary);
}

.ex-line {
    display: block;
    width: 26px;
    height: 2px;
    background: var(--clr-text-dim);
    border-radius: 2px;
}

.example-eq {
    color: var(--clr-text-dim);
    font-size: 1rem;
}

.example-unknown {
    color: var(--clr-accent);
    font-weight: 700;
}


/* ============================================
   FAQ
   ============================================ */
.faq-section {
    max-width: 700px;
    margin: 0 auto;
}

.faq-list {
    display: flex;
    flex-direction: column;
    gap: var(--space-sm);
}

.faq-item {
    background: var(--clr-surface);
    border: 1px solid var(--clr-border);
    border-radius: var(--radius-lg);
    overflow: hidden;
    box-shadow: var(--shadow-xs);
    transition: all var(--dur-md);
}

.faq-item[open] {
    border-color: var(--clr-border-hover);
    box-shadow: var(--shadow-sm);
}

.faq-question {
    padding: var(--space-lg) var(--space-xl);
    font-weight: 600;
    font-size: 0.95rem;
    cursor: pointer;
    list-style: none;
    display: flex;
    align-items: center;
    justify-content: space-between;
    color: var(--clr-text);
    transition: color var(--dur-fast);
}

.faq-question::-webkit-details-marker {
    display: none;
}

.faq-question::after {
    content: '+';
    font-size: 1.3rem;
    font-weight: 300;
    color: var(--clr-text-dim);
    transition: all var(--dur-md) var(--ease-out);
    flex-shrink: 0;
    margin-left: var(--space-md);
    width: 28px;
    height: 28px;
    display: flex;
    align-items: center;
    justify-content: center;
    border-radius: 50%;
    background: var(--clr-surface-2);
}

.faq-item[open] .faq-question::after {
    transform: rotate(45deg);
    color: var(--clr-accent);
    background: var(--clr-accent-soft);
}

.faq-question:hover {
    color: var(--clr-accent);
}

.faq-answer {
    padding: 0 var(--space-xl) var(--space-lg);
    font-size: 0.925rem;
    color: var(--clr-text-muted);
    line-height: 1.8;
    animation: slideDown 0.25s var(--ease-out);
}

.faq-answer strong {
    color: var(--clr-text-secondary);
}

@keyframes slideDown {
    from { opacity: 0; transform: translateY(-6px); }
    to   { opacity: 1; transform: translateY(0); }
}


/* ============================================
   FOOTER (original)
   ============================================ */
.footer {
    padding: var(--space-2xl) var(--space-xl);
    border-top: 1px solid var(--clr-border);
    margin-top: 0;
}

.footer-inner {
    max-width: 1100px;
    margin: 0 auto;
    display: flex;
    align-items: center;
    justify-content: space-between;
}

.footer-brand {
    display: flex;
    align-items: center;
    gap: var(--space-sm);
    font-weight: 700;
    font-size: 1rem;
    color: var(--clr-text);
}

.footer-copy {
    font-size: 0.85rem;
    color: var(--clr-text-dim);
}


/* ============================================
   ANIMATIONS
   ============================================ */
@keyframes fadeInUp {
    from {
        opacity: 0;
        transform: translateY(20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

@keyframes slideUp {
    from {
        opacity: 0;
        transform: translateY(10px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

/* Scroll reveal */
.reveal {
    opacity: 0;
    transform: translateY(20px);
    transition: opacity 0.5s var(--ease-out), transform 0.5s var(--ease-out);
}

.reveal.visible {
    opacity: 1;
    transform: translateY(0);
}


/* ============================================
   RESPONSIVE (original)
   ============================================ */

/* Tablet */
@media (max-width: 1024px) {
    .hero {
        flex-direction: column;
        text-align: center;
        gap: var(--space-2xl);
        padding-top: 130px;
    }

    .hero-content {
        max-width: 100%;
    }

    .examples-grid {
        grid-template-columns: repeat(2, 1fr);
    }
}

/* Mobile */
@media (max-width: 768px) {
    :root {
        --space-xl: 1.5rem;
        --space-2xl: 2rem;
        --space-3xl: 2.5rem;
        --space-4xl: 3rem;
    }

    .nav {
        display: none;
        position: absolute;
        top: 100%;
        left: 0;
        right: 0;
        flex-direction: column;
        background: var(--nav-mobile-bg);
        backdrop-filter: blur(16px);
        -webkit-backdrop-filter: blur(16px);
        border-bottom: 1px solid var(--clr-border);
        padding: var(--space-md);
        gap: 2px;
    }

    .nav.open {
        display: flex;
    }

    .mobile-menu-btn {
        display: flex;
    }

    .hero-title {
        font-size: clamp(2rem, 8vw, 2.75rem);
    }

    .formula-display {
        padding: var(--space-xl);
        gap: var(--space-lg);
    }

    .formula-num,
    .formula-den {
        font-size: 1.6rem;
    }

    .formula-line {
        width: 36px;
    }

    .formula-equals {
        font-size: 1.6rem;
    }

    .steps-grid {
        grid-template-columns: 1fr;
        gap: var(--space-md);
    }

    .proportion-grid {
        gap: var(--space-md);
    }

    .calc-input {
        font-size: 1.15rem;
        padding: 14px var(--space-md);
    }

    .equals-sign {
        font-size: 1.5rem;
    }

    .calc-actions {
        flex-direction: column;
        align-items: stretch;
    }

    .btn {
        justify-content: center;
    }

    .result-value {
        font-size: 1.4rem;
    }

    .footer-inner {
        flex-direction: column;
        gap: var(--space-md);
        text-align: center;
    }
}

/* Mobile Small */
@media (max-width: 480px) {
    .header {
        padding: var(--space-sm) var(--space-md);
    }

    section {
        padding: var(--space-2xl) var(--space-md);
    }

    .calculator-card {
        padding: var(--space-lg);
        border-radius: var(--radius-xl);
    }

    .proportion-grid {
        flex-direction: column;
        gap: var(--space-lg);
    }

    .fraction-group {
        max-width: 100%;
        width: 100%;
    }

    .equals-sign {
        transform: rotate(90deg);
        font-size: 1.5rem;
    }

    .fraction-divider {
        width: 60%;
    }

    .calc-instruction {
        font-size: 0.8rem;
        padding: var(--space-sm) var(--space-md);
    }

    .examples-grid {
        grid-template-columns: 1fr 1fr;
        gap: var(--space-sm);
    }

    .example-card {
        padding: var(--space-md);
    }

    .math-sym { opacity: 0.12; }

    .hero {
        padding-top: 100px;
    }
}


/* ============================================
   NEW CONTENT SECTIONS (appended below original)
   ============================================ */

/* Container */
.container {
    max-width: 1100px;
    margin: 0 auto;
}

/* Section tag badge */
.section-tag {
    display: inline-block;
    padding: 4px 14px;
    background: var(--clr-accent-soft);
    border: 1px solid rgba(99, 102, 241, 0.12);
    border-radius: var(--radius-full);
    font-size: 0.75rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.08em;
    color: var(--clr-accent);
    margin-bottom: var(--space-md);
}

/* Widen section-header for new sections */
.content-section .section-header {
    max-width: 640px;
}

/* Content sections with alternating background */
.content-section {
    padding: var(--space-4xl) var(--space-xl);
}

.content-section.alt-bg {
    background: var(--clr-bg-alt);
}

/* Hero stats bar */
.hero-stats {
    display: flex;
    gap: var(--space-xl);
    flex-wrap: wrap;
    animation: fadeInUp 0.6s var(--ease-out) 0.4s both;
}

.hero-stat {
    text-align: center;
}

.hero-stat-value {
    font-family: var(--font-mono);
    font-size: 1.2rem;
    font-weight: 700;
    color: var(--clr-accent);
    display: block;
}

.hero-stat-label {
    font-size: 0.75rem;
    color: var(--clr-text-muted);
    margin-top: 2px;
}

.hero-stat-divider {
    width: 1px;
    background: var(--clr-border);
    align-self: stretch;
}

/* ── Content text blocks ── */
.content-text-block {
    font-size: 1rem;
    color: var(--clr-text-secondary);
    line-height: 1.8;
}

.content-text-block p {
    margin-bottom: var(--space-lg);
}

.content-text-block p:last-child {
    margin-bottom: 0;
}

.content-text-block strong {
    color: var(--clr-text);
    font-weight: 600;
}

.centered-text {
    max-width: 800px;
    margin: 0 auto;
}

/* ── Content grid (text + visual side-by-side) ── */
.content-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: var(--space-3xl);
    align-items: center;
}

.formula-content-grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: var(--space-3xl);
    align-items: start;
}

/* ── Flow diagram ── */
.flow-diagram-container {
    display: flex;
    justify-content: center;
}

.interactive-flow-diagram {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: var(--space-md);
    position: relative;
}

.ifd-flow-wrapper {
    display: flex;
    align-items: center;
    gap: 0;
}

.ifd-column {
    display: flex;
    flex-direction: column;
    gap: var(--space-md);
}

.ifd-node {
    padding: var(--space-lg) var(--space-xl);
    border-radius: var(--radius-lg);
    text-align: center;
    border: 2px solid var(--clr-border);
    background: var(--clr-surface);
    box-shadow: var(--shadow-card);
    transition: all 0.4s cubic-bezier(0.34, 1.56, 0.64, 1);
    position: relative;
    overflow: hidden;
    user-select: none;
    min-width: 130px;
}

.ifd-node:hover {
    transform: translateY(-3px);
    box-shadow: var(--shadow-card-hover);
}

.ifd-node-icon { font-size: 1.5rem; margin-bottom: var(--space-sm); transition: transform 0.3s var(--ease-spring); }
.ifd-node:hover .ifd-node-icon { transform: scale(1.15); }
.ifd-node-title { font-weight: 700; font-size: 0.95rem; color: var(--clr-text); margin-bottom: 2px; }
.ifd-node-sub { font-size: 0.82rem; color: var(--clr-text-muted); font-family: var(--font-mono); }

/* Value display — always visible, prominent */
.ifd-node-value {
    font-family: var(--font-mono);
    font-size: 1.1rem;
    font-weight: 700;
    color: var(--clr-accent);
    margin-top: var(--space-sm);
    padding-top: var(--space-xs);
    border-top: 1px solid var(--clr-border);
    letter-spacing: 0.02em;
    opacity: 0;
    transition: opacity 0.5s var(--ease-out), transform 0.5s var(--ease-out);
    transform: translateY(4px);
}

.ifd-node.ifd-active .ifd-node-value {
    opacity: 1;
    transform: translateY(0);
}

.ifd-result-value {
    color: var(--clr-success);
    font-size: 1.25rem;
    font-weight: 800;
    letter-spacing: 0.03em;
}

/* Pulse ring animation on active nodes */
.ifd-node-pulse {
    position: absolute;
    inset: -4px;
    border-radius: inherit;
    border: 2px solid var(--clr-accent);
    opacity: 0;
    pointer-events: none;
    animation: none;
}

.ifd-node.ifd-active .ifd-node-pulse {
    animation: ifd-pulse-ring 1.5s ease-out infinite;
}

@keyframes ifd-pulse-ring {
    0% { transform: scale(1); opacity: 0.6; }
    100% { transform: scale(1.15); opacity: 0; }
}

/* Active state styling per node type */
.ifd-node.ifd-active {
    border-color: var(--clr-accent);
    box-shadow: 0 0 20px rgba(99, 102, 241, 0.2), var(--shadow-card-hover);
    transform: translateY(-3px) scale(1.02);
}

.ifd-engine-node { border-color: rgba(99, 102, 241, 0.2); background: var(--clr-accent-softer); }
.ifd-engine-node.ifd-active {
    border-color: var(--clr-accent);
    box-shadow: 0 0 24px rgba(99, 102, 241, 0.3), var(--shadow-card-hover);
}

.ifd-output-node { border-color: rgba(22, 163, 74, 0.2); background: rgba(22, 163, 74, 0.04); }
.ifd-output-node.ifd-active {
    border-color: var(--clr-success);
    box-shadow: 0 0 24px rgba(22, 163, 74, 0.2), var(--shadow-card-hover);
}
.ifd-output-node.ifd-active .ifd-node-pulse {
    border-color: var(--clr-success);
}

/* Completed state */
.ifd-node.ifd-completed {
    border-color: rgba(22, 163, 74, 0.3);
    background: rgba(22, 163, 74, 0.04);
}

/* Gear spin effect */
.ifd-gear-spin {
    position: absolute;
    top: 8px;
    right: 8px;
    width: 18px;
    height: 18px;
    opacity: 0;
    transition: opacity 0.3s;
}
.ifd-gear-spin::after {
    content: '⚙';
    font-size: 14px;
    display: block;
    animation: ifd-spin 2s linear infinite;
    opacity: 0.3;
}
.ifd-engine-node.ifd-active .ifd-gear-spin {
    opacity: 1;
}
@keyframes ifd-spin {
    from { transform: rotate(0deg); }
    to { transform: rotate(360deg); }
}

/* SVG Connector arrows */
.ifd-connector {
    display: flex;
    align-items: center;
    padding: 0 var(--space-xs);
    opacity: 0.4;
    transition: opacity 0.5s var(--ease-out);
}

.ifd-connector.ifd-active {
    opacity: 1;
}

.ifd-svg-arrow {
    width: 60px;
    height: 80px;
}

#ifd-connector-2 .ifd-svg-arrow {
    height: 10px;
}

.ifd-arrow-path {
    stroke-dasharray: 4 4;
    animation: ifd-dash 1s linear infinite paused;
}

.ifd-connector.ifd-active .ifd-arrow-path {
    animation-play-state: running;
    stroke-dasharray: none;
}

@keyframes ifd-dash {
    to { stroke-dashoffset: -8; }
}

.ifd-flow-dot {
    opacity: 0;
    transition: opacity 0.3s;
}

.ifd-connector.ifd-active .ifd-flow-dot {
    opacity: 1;
}



/* ── Formula box ── */
.formula-box {
    background: var(--clr-surface);
    border: 1px solid var(--clr-border);
    border-radius: var(--radius-lg);
    padding: var(--space-xl);
    margin: var(--space-lg) 0;
    box-shadow: var(--shadow-card);
}

.formula-row {
    display: flex; align-items: center; gap: var(--space-md);
    padding: var(--space-sm) 0;
    border-bottom: 1px solid var(--clr-border);
}
.formula-row:last-child { border-bottom: none; }
.formula-label { font-weight: 700; font-size: 0.9rem; color: var(--clr-accent); min-width: 100px; font-family: var(--font-mono); }
.formula-expression { font-family: var(--font-mono); font-size: 0.95rem; color: var(--clr-text); font-weight: 500; }

/* ── Formula walkthrough card ── */
.formula-walkthrough-card {
    background: var(--clr-surface);
    border: 1px solid var(--clr-border);
    border-radius: var(--radius-xl);
    padding: var(--space-xl);
    box-shadow: var(--shadow-lg);
}
.fw-title { font-weight: 700; font-size: 1rem; color: var(--clr-text); margin-bottom: var(--space-lg); text-align: center; }
.fw-inputs { display: flex; align-items: center; justify-content: center; gap: var(--space-sm); margin-bottom: var(--space-xl); flex-wrap: wrap; }
.fw-input-group { display: flex; flex-direction: column; align-items: center; gap: 4px; }
.fw-input-group label { font-size: 0.7rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.08em; color: var(--clr-text-dim); }

.fw-input {
    width: 60px; padding: 10px 8px;
    background: var(--clr-bg-warm); border: 2px solid var(--clr-surface-3);
    border-radius: var(--radius-sm); font-size: 1.1rem; font-family: var(--font-mono);
    font-weight: 600; text-align: center; color: var(--clr-text);
    transition: all var(--dur-md) var(--ease-out);
}
.fw-input:focus { border-color: var(--clr-accent-light); background: #fff; box-shadow: 0 0 0 3px var(--clr-accent-soft); }
.fw-input::placeholder { color: var(--clr-accent); font-weight: 400; }
.fw-divider, .fw-equals { font-size: 1.3rem; font-weight: 300; color: var(--clr-text-dim); margin-top: 18px; }

.fw-steps { display: flex; flex-direction: column; gap: var(--space-sm); margin-bottom: var(--space-lg); }
.fw-step { display: flex; align-items: flex-start; gap: var(--space-md); padding: var(--space-md); background: var(--clr-bg-warm); border-radius: var(--radius-md); border: 1px solid var(--clr-border); }
.fw-step-num { width: 28px; height: 28px; display: flex; align-items: center; justify-content: center; font-family: var(--font-mono); font-size: 0.8rem; font-weight: 700; background: var(--clr-accent-soft); color: var(--clr-accent); border-radius: var(--radius-sm); flex-shrink: 0; }
.fw-step-content { flex: 1; }
.fw-step-label { font-size: 0.75rem; font-weight: 600; text-transform: uppercase; letter-spacing: 0.06em; color: var(--clr-text-muted); margin-bottom: 2px; }
.fw-step-calc { font-family: var(--font-mono); font-size: 0.85rem; color: var(--clr-text); }

.fw-result { text-align: center; padding: var(--space-md); background: var(--clr-success-bg); border: 1px solid var(--clr-success-border); border-radius: var(--radius-md); }
.fw-result-label { font-size: 0.8rem; font-weight: 600; color: var(--clr-text-muted); margin-right: var(--space-sm); }
.fw-result-value { font-family: var(--font-mono); font-weight: 700; color: var(--clr-success); font-size: 1rem; }

/* ── Types of proportions ── */
.types-grid {
    display: grid; grid-template-columns: repeat(3, 1fr); gap: var(--space-lg);
}

.type-card {
    padding: var(--space-2xl) var(--space-xl);
    background: var(--clr-surface); border: 1px solid var(--clr-border);
    border-radius: var(--radius-xl); box-shadow: var(--shadow-card);
    transition: all var(--dur-md) var(--ease-out);
}
.type-card:hover { transform: translateY(-4px); box-shadow: var(--shadow-card-hover); }
.type-icon { font-size: 2rem; margin-bottom: var(--space-md); }
.type-card h3 { font-size: 1.1rem; font-weight: 700; margin-bottom: var(--space-sm); color: var(--clr-text); }
.type-card p { font-size: 0.9rem; color: var(--clr-text-muted); line-height: 1.7; margin-bottom: var(--space-md); }

.type-example {
    padding: var(--space-md); background: var(--clr-accent-softer);
    border: 1px solid rgba(99, 102, 241, 0.08); border-radius: var(--radius-md);
    font-size: 0.85rem; line-height: 1.6; color: var(--clr-text-secondary);
}
.type-example-label { font-weight: 700; color: var(--clr-accent); display: block; margin-bottom: 4px; font-size: 0.75rem; text-transform: uppercase; letter-spacing: 0.06em; }

/* ── Worked examples accordion ── */
.worked-examples-list { display: flex; flex-direction: column; gap: var(--space-md); max-width: 800px; margin: 0 auto; }

.worked-example-card {
    background: var(--clr-surface); border: 1px solid var(--clr-border);
    border-radius: var(--radius-xl); overflow: hidden;
    box-shadow: var(--shadow-card); transition: all var(--dur-md);
}
.worked-example-card.open { border-color: var(--clr-border-hover); box-shadow: var(--shadow-md); }

.worked-example-header {
    width: 100%; display: flex; align-items: center; justify-content: space-between;
    padding: var(--space-lg) var(--space-xl); text-align: left; transition: background var(--dur-fast);
}
.worked-example-header:hover { background: var(--clr-accent-softer); }

.worked-example-header-left { display: flex; align-items: center; gap: var(--space-md); }
.worked-example-icon { font-size: 1.5rem; flex-shrink: 0; }
.worked-example-header h3 { font-size: 1rem; font-weight: 700; color: var(--clr-text); margin-bottom: 2px; }
.worked-example-subtitle { font-size: 0.85rem; color: var(--clr-text-muted); }

.worked-example-chevron { color: var(--clr-text-dim); transition: transform var(--dur-md) var(--ease-out); flex-shrink: 0; }
.worked-example-card.open .worked-example-chevron { transform: rotate(180deg); color: var(--clr-accent); }

.worked-example-body { display: none; padding: 0 var(--space-xl) var(--space-xl); animation: slideDown 0.25s var(--ease-out); }
.worked-example-card.open .worked-example-body { display: block; }
.worked-example-body > p { font-size: 0.95rem; color: var(--clr-text-secondary); line-height: 1.7; margin-bottom: var(--space-lg); }

.worked-example-steps { display: flex; flex-direction: column; gap: var(--space-sm); margin-bottom: var(--space-lg); }
.we-step { display: flex; align-items: center; gap: var(--space-md); padding: var(--space-md); background: var(--clr-bg-warm); border-radius: var(--radius-md); border: 1px solid var(--clr-border); font-size: 0.9rem; color: var(--clr-text-secondary); }
.we-step strong { color: var(--clr-text); font-family: var(--font-mono); }
.we-step-num { width: 28px; height: 28px; display: flex; align-items: center; justify-content: center; font-family: var(--font-mono); font-size: 0.8rem; font-weight: 700; background: var(--clr-accent-soft); color: var(--clr-accent); border-radius: var(--radius-sm); flex-shrink: 0; }

.we-result { padding: var(--space-md) var(--space-lg); background: var(--clr-success-bg); border: 1px solid var(--clr-success-border); border-radius: var(--radius-md); margin-bottom: var(--space-md); }
.we-result-label { font-size: 0.8rem; font-weight: 600; color: var(--clr-text-muted); margin-right: var(--space-sm); }
.we-result-value { font-family: var(--font-mono); font-weight: 700; color: var(--clr-success); font-size: 1.05rem; }
.we-verify { font-size: 0.85rem; color: var(--clr-text-muted); padding: var(--space-md); background: var(--clr-accent-softer); border-radius: var(--radius-md); border: 1px solid rgba(99, 102, 241, 0.08); }
.we-verify strong { color: var(--clr-text-secondary); }

/* ── Applications grid ── */
.applications-grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: var(--space-lg); }

.app-card {
    padding: var(--space-2xl) var(--space-xl); background: var(--clr-surface);
    border: 1px solid var(--clr-border); border-radius: var(--radius-xl);
    box-shadow: var(--shadow-card); transition: all var(--dur-md) var(--ease-out);
}
.app-card:hover { transform: translateY(-4px); box-shadow: var(--shadow-card-hover); }
.app-card-icon { font-size: 2rem; margin-bottom: var(--space-md); }
.app-card h3 { font-size: 1rem; font-weight: 700; margin-bottom: var(--space-sm); color: var(--clr-text); }
.app-card p { font-size: 0.88rem; color: var(--clr-text-muted); line-height: 1.7; }

/* ── Comparison sections ── */
.comparison-section { display: flex; gap: var(--space-lg); max-width: 900px; margin: 0 auto; align-items: stretch; }

.comparison-column {
    flex: 1; background: var(--clr-surface); border: 1px solid var(--clr-border);
    border-radius: var(--radius-xl); overflow: hidden; box-shadow: var(--shadow-card);
}
.comparison-col-header { display: flex; align-items: center; gap: var(--space-md); padding: var(--space-lg) var(--space-xl); border-bottom: 1px solid var(--clr-border); }
.comparison-col-header h3 { font-size: 1.05rem; font-weight: 700; }
.comp-icon { font-size: 1.3rem; flex-shrink: 0; }

.ratio-header { background: rgba(99, 102, 241, 0.06); }
.proportion-header { background: rgba(22, 163, 74, 0.06); }
.bad-header { background: rgba(220, 38, 38, 0.06); }
.bad-header .comp-icon { color: var(--clr-error); }
.good-header { background: rgba(22, 163, 74, 0.06); }
.good-header .comp-icon { color: var(--clr-success); }

.comparison-list { list-style: none; padding: var(--space-lg) var(--space-xl); }
.comparison-list li { padding: var(--space-sm) 0; font-size: 0.9rem; color: var(--clr-text-secondary); line-height: 1.6; position: relative; padding-left: var(--space-xl); }
.comparison-list li::before { content: '•'; position: absolute; left: 0; color: var(--clr-text-dim); }
.comparison-list li.comp-bad::before { content: '✗'; color: var(--clr-error); }
.comparison-list li.comp-good::before { content: '✓'; color: var(--clr-success); }
.comparison-list li strong { color: var(--clr-text); }

.comparison-vs-divider { display: flex; align-items: center; justify-content: center; flex-shrink: 0; }
.comparison-vs-divider span { font-weight: 800; font-size: 0.85rem; color: var(--clr-text-dim); padding: var(--space-sm) var(--space-md); background: var(--clr-surface-2); border-radius: var(--radius-full); border: 1px solid var(--clr-border); }

.comparison-summary { margin-top: var(--space-2xl); padding: var(--space-lg) var(--space-xl); background: var(--clr-accent-softer); border: 1px solid rgba(99, 102, 241, 0.1); border-radius: var(--radius-lg); }
.comparison-summary p { margin-bottom: 0; }

/* ── Proportion table ── */
.table-wrapper { max-width: 800px; margin: 0 auto; overflow-x: auto; -webkit-overflow-scrolling: touch; }

.proportion-ref-table { width: 100%; border-collapse: collapse; background: var(--clr-surface); border-radius: var(--radius-lg); overflow: hidden; box-shadow: var(--shadow-card); border: 1px solid var(--clr-border); }
.proportion-ref-table thead { background: var(--clr-accent-soft); }
.proportion-ref-table th { padding: var(--space-md) var(--space-lg); text-align: left; font-size: 0.8rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.06em; color: var(--clr-accent); border-bottom: 2px solid rgba(99, 102, 241, 0.15); }
.proportion-ref-table td { padding: var(--space-md) var(--space-lg); font-size: 0.9rem; color: var(--clr-text-secondary); border-bottom: 1px solid var(--clr-border); }
.proportion-ref-table td:first-child { font-family: var(--font-mono); font-weight: 600; color: var(--clr-text); }
.proportion-ref-table td:nth-child(2) { font-family: var(--font-mono); font-size: 0.85rem; }
.proportion-ref-table tbody tr:last-child td { border-bottom: none; }
.proportion-ref-table tbody tr:hover { background: var(--clr-accent-softer); }

/* ── Privacy section ── */
.privacy-grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: var(--space-lg); }

.privacy-card {
    padding: var(--space-2xl) var(--space-xl); background: var(--clr-surface);
    border: 1px solid var(--clr-border); border-radius: var(--radius-xl);
    box-shadow: var(--shadow-card); text-align: center; transition: all var(--dur-md) var(--ease-out);
}
.privacy-card:hover { transform: translateY(-4px); box-shadow: var(--shadow-card-hover); }
.privacy-icon { font-size: 2rem; margin-bottom: var(--space-md); }
.privacy-card h3 { font-size: 1rem; font-weight: 700; margin-bottom: var(--space-sm); color: var(--clr-text); }
.privacy-card p { font-size: 0.88rem; color: var(--clr-text-muted); line-height: 1.7; }

/* ── Blog section ── */
.blog-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: var(--space-lg);
}

.blog-card {
    background: var(--clr-surface);
    border: 1px solid var(--clr-border);
    border-radius: var(--radius-xl);
    overflow: hidden;
    box-shadow: var(--shadow-card);
    transition: all var(--dur-md) var(--ease-out);
    display: flex;
    flex-direction: column;
}

.blog-card:hover {
    transform: translateY(-6px);
    box-shadow: var(--shadow-card-hover);
    border-color: var(--clr-border-hover);
}

.blog-card-image {
    position: relative;
    overflow: hidden;
}

.blog-card-placeholder {
    width: 100%;
    height: 160px;
    background: linear-gradient(135deg, var(--clr-accent-soft), rgba(139, 92, 246, 0.08));
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all var(--dur-md) var(--ease-out);
    overflow: hidden;
}

.blog-card:hover .blog-card-placeholder {
    background: linear-gradient(135deg, rgba(99, 102, 241, 0.12), rgba(139, 92, 246, 0.15));
}

.blog-card-placeholder img {
    width: 100%;
    height: 100%;
    object-fit: cover;
    transition: transform var(--dur-md) var(--ease-out);
}

.blog-card:hover .blog-card-placeholder img {
    transform: scale(1.06);
}

.blog-card-placeholder span {
    font-size: 3rem;
    opacity: 0.7;
    transition: transform var(--dur-md) var(--ease-spring);
}

.blog-card:hover .blog-card-placeholder span {
    transform: scale(1.15);
}

.blog-card-category {
    position: absolute;
    top: var(--space-md);
    left: var(--space-md);
    padding: 4px 12px;
    background: var(--clr-surface);
    border: 1px solid var(--clr-border);
    border-radius: var(--radius-full);
    font-size: 0.7rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.06em;
    color: var(--clr-accent);
    box-shadow: var(--shadow-sm);
}

.blog-card-content {
    padding: var(--space-xl);
    display: flex;
    flex-direction: column;
    flex: 1;
}

.blog-card-meta {
    display: flex;
    align-items: center;
    gap: var(--space-md);
    margin-bottom: var(--space-sm);
}

.blog-card-date,
.blog-card-read {
    font-size: 0.75rem;
    color: var(--clr-text-dim);
    font-weight: 500;
}

.blog-card-date::after {
    content: '';
    display: inline-block;
    width: 3px;
    height: 3px;
    background: var(--clr-text-dim);
    border-radius: 50%;
    margin-left: var(--space-md);
    vertical-align: middle;
}

.blog-card-title {
    font-size: 1.05rem;
    font-weight: 700;
    line-height: 1.4;
    color: var(--clr-text);
    margin-bottom: var(--space-sm);
    transition: color var(--dur-fast);
}

.blog-card:hover .blog-card-title {
    color: var(--clr-accent);
}

.blog-card-excerpt {
    font-size: 0.88rem;
    color: var(--clr-text-muted);
    line-height: 1.65;
    margin-bottom: var(--space-lg);
    flex: 1;
}

.blog-card-link {
    display: inline-flex;
    align-items: center;
    gap: 6px;
    font-size: 0.85rem;
    font-weight: 600;
    color: var(--clr-accent);
    transition: all var(--dur-fast);
}

.blog-card-link:hover {
    gap: 10px;
}

.blog-card-link svg {
    transition: transform var(--dur-md) var(--ease-out);
}

.blog-card-link:hover svg {
    transform: translateX(3px);
}

/* Blog card expandable body */
.blog-card-body {
    display: none;
    padding: 0 var(--space-xl) var(--space-xl);
    border-top: 1px solid var(--clr-border);
    animation: slideDown 0.3s var(--ease-out);
}

.blog-card.open .blog-card-body {
    display: block;
}

.blog-card-body p {
    font-size: 0.92rem;
    color: var(--clr-text-secondary);
    line-height: 1.8;
    margin-bottom: var(--space-md);
}

.blog-card-body p:last-child {
    margin-bottom: 0;
}

.blog-card-body strong {
    color: var(--clr-text);
    font-weight: 600;
}

.blog-card.open {
    grid-column: 1 / -1;
    border-color: var(--clr-accent-light);
    box-shadow: var(--shadow-lg);
}

.blog-card.open .blog-card-link svg {
    transform: rotate(90deg);
}

.blog-card.open .blog-card-link {
    color: var(--clr-text-muted);
}

/* ── Enhanced footer ── */
.footer-grid {
    display: grid; grid-template-columns: 1.5fr 1fr 1fr 1fr;
    gap: var(--space-2xl); margin-bottom: var(--space-2xl);
}
.footer-tagline { font-size: 0.85rem; color: var(--clr-text-muted); line-height: 1.6; }
.footer-column h4 { font-size: 0.8rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.08em; color: var(--clr-text); margin-bottom: var(--space-md); }
.footer-links { list-style: none; }
.footer-links li { margin-bottom: var(--space-sm); }
.footer-links a { font-size: 0.88rem; color: var(--clr-text-muted); transition: color var(--dur-fast); }
.footer-links a:hover { color: var(--clr-accent); }
.footer-bottom { padding-top: var(--space-xl); border-top: 1px solid var(--clr-border); text-align: center; }

/* ── New sections responsive ── */
@media (max-width: 1024px) {
    .hero-stats { justify-content: center; }
    .content-grid, .formula-content-grid { grid-template-columns: 1fr; gap: var(--space-2xl); }
    .types-grid, .applications-grid, .privacy-grid, .blog-grid { grid-template-columns: repeat(2, 1fr); }
    .comparison-section { flex-direction: column; }
    .comparison-vs-divider { padding: var(--space-sm) 0; }
    .footer-grid { grid-template-columns: 1fr 1fr; }
}

@media (max-width: 768px) {
    .types-grid, .applications-grid, .privacy-grid, .blog-grid { grid-template-columns: 1fr; }
    .footer-grid { grid-template-columns: 1fr; gap: var(--space-lg); }
    .footer-inner { text-align: center; }
    .footer-brand { justify-content: center; }
    .ifd-flow-wrapper { flex-direction: column; }
    .ifd-connector { transform: rotate(90deg); padding: var(--space-xs) 0; }
    .ifd-svg-arrow { width: 40px; }
    #ifd-connector-2 .ifd-svg-arrow { height: 10px; }
    .fw-inputs { gap: 4px; }
    .fw-input { width: 50px; padding: 8px 4px; font-size: 0.95rem; }
}

@media (max-width: 480px) {
    .worked-example-header { padding: var(--space-md); }
    .worked-example-body { padding: 0 var(--space-md) var(--space-md); }
    .worked-example-header-left { gap: var(--space-sm); }
    .worked-example-header h3 { font-size: 0.9rem; }
    .formula-box { padding: var(--space-md); }
    .formula-row { flex-direction: column; gap: var(--space-xs); align-items: flex-start; }
}


/* ============================================
   BLOG CARD LINKS (stretched, whole card clickable)
   ============================================ */
.blog-card { position: relative; transition: transform var(--dur-md) var(--ease-out), box-shadow var(--dur-md) var(--ease-out), border-color var(--dur-md); }
.blog-card-image { display: block; text-decoration: none; }
.blog-card-link { background: none; border: none; cursor: pointer; padding: 0; margin-top: auto; text-decoration: none; }
.blog-card-link::after { content: ''; position: absolute; inset: 0; z-index: 1; }
.blog-card:hover { transform: translateY(-4px); box-shadow: var(--shadow-card-hover); border-color: var(--clr-accent-light); }

/* ============================================
   BLOG ARTICLE PAGE
   ============================================ */
.article-main { max-width: 800px; margin: 0 auto; padding: calc(72px + var(--space-3xl)) var(--space-lg) var(--space-4xl); position: relative; z-index: 1; }
.article-back { display: inline-flex; align-items: center; gap: 6px; color: var(--clr-text-muted); font-size: 0.85rem; font-weight: 600; text-decoration: none; margin-bottom: var(--space-lg); transition: color var(--dur-fast), gap var(--dur-fast); }
.article-back:hover { color: var(--clr-accent); gap: 10px; }
.article-category { display: inline-block; padding: 4px 14px; border-radius: var(--radius-full); font-size: 0.7rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.06em; color: var(--clr-accent); background: var(--clr-accent-soft); margin-bottom: var(--space-md); }
.article-title { font-size: clamp(1.8rem, 5vw, 2.8rem); font-weight: 800; line-height: 1.15; letter-spacing: -0.02em; color: var(--clr-text); margin-bottom: var(--space-md); }
.article-meta { display: flex; align-items: center; flex-wrap: wrap; gap: var(--space-md); color: var(--clr-text-dim); font-size: 0.85rem; font-weight: 500; margin-bottom: var(--space-xl); }
.article-meta span { display: inline-flex; align-items: center; gap: 6px; }
.article-cover { display: flex; align-items: center; justify-content: center; height: 360px; border-radius: var(--radius-xl); font-size: 5rem; background: linear-gradient(135deg, var(--clr-accent-soft), var(--clr-surface-2)); border: 1px solid var(--clr-border); margin-bottom: var(--space-2xl); padding: 0; overflow: hidden; }
.article-cover img { width: 100%; height: 100%; object-fit: cover; }
.article-content { color: var(--clr-text-secondary); font-size: 1.05rem; line-height: 1.85; }
.article-content > p:first-of-type { font-size: 1.18rem; color: var(--clr-text); line-height: 1.7; }
.article-content h2 { font-size: 1.6rem; font-weight: 700; color: var(--clr-text); margin: var(--space-2xl) 0 var(--space-md); letter-spacing: -0.01em; line-height: 1.25; }
.article-content h3 { font-size: 1.2rem; font-weight: 700; color: var(--clr-text); margin: var(--space-xl) 0 var(--space-sm); }
.article-content p { margin-bottom: var(--space-lg); }
.article-content ul, .article-content ol { margin: 0 0 var(--space-lg) 1.4rem; }
.article-content li { margin-bottom: var(--space-sm); }
.article-content strong { color: var(--clr-text); font-weight: 700; }
.article-content a { color: var(--clr-accent); text-decoration: underline; text-underline-offset: 2px; }
.article-content blockquote { border-left: 3px solid var(--clr-accent); background: var(--clr-accent-softer); padding: var(--space-md) var(--space-lg); border-radius: 0 var(--radius-md) var(--radius-md) 0; margin: 0 0 var(--space-lg); color: var(--clr-text); font-style: italic; }
.article-content .formula-callout { background: var(--clr-surface-2); border: 1px solid var(--clr-border); border-radius: var(--radius-lg); padding: var(--space-lg); margin: 0 0 var(--space-lg); font-family: var(--font-mono); font-size: 1.05rem; color: var(--clr-text); text-align: center; }
.article-content table { width: 100%; border-collapse: collapse; margin: 0 0 var(--space-lg); font-size: 0.95rem; }
.article-content th, .article-content td { padding: 10px 14px; border: 1px solid var(--clr-border); text-align: left; }
.article-content th { background: var(--clr-surface-2); color: var(--clr-text); font-weight: 700; }
.article-cta { margin: var(--space-3xl) 0 var(--space-2xl); padding: var(--space-2xl); border-radius: var(--radius-xl); text-align: center; background: linear-gradient(135deg, var(--clr-accent-soft), var(--clr-surface-2)); border: 1px solid var(--clr-border); }
.article-cta h3 { font-size: 1.35rem; font-weight: 700; color: var(--clr-text); margin-bottom: var(--space-sm); }
.article-cta p { color: var(--clr-text-muted); margin-bottom: var(--space-lg); }
.article-cta .hero-cta { display: inline-flex; }
.article-related { margin-top: var(--space-3xl); }
.article-related h3 { font-size: 1.3rem; font-weight: 700; color: var(--clr-text); margin-bottom: var(--space-lg); }
.related-grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(220px, 1fr)); gap: var(--space-lg); }
.related-card { display: block; text-decoration: none; padding: var(--space-lg); background: var(--clr-surface); border: 1px solid var(--clr-border); border-radius: var(--radius-lg); transition: transform var(--dur-md) var(--ease-out), box-shadow var(--dur-md), border-color var(--dur-md); }
.related-card:hover { transform: translateY(-3px); box-shadow: var(--shadow-card-hover); border-color: var(--clr-accent-light); }
.related-card .rc-cat { font-size: 0.68rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.06em; color: var(--clr-accent); }
.related-card .rc-title { font-size: 0.98rem; font-weight: 700; color: var(--clr-text); line-height: 1.4; margin-top: 6px; }

/* ============================================
   READING PROGRESS BAR + BACK TO TOP
   ============================================ */
.reading-progress { position: fixed; top: 0; left: 0; height: 3px; width: 0%; background: var(--gradient-primary); z-index: 200; transition: width 0.1s linear; box-shadow: 0 0 8px rgba(99,102,241,0.4); }
.back-to-top { position: fixed; bottom: 24px; right: 24px; z-index: 150; width: 46px; height: 46px; border-radius: 50%; border: 1px solid var(--clr-border); background: var(--clr-surface); color: var(--clr-accent); cursor: pointer; display: flex; align-items: center; justify-content: center; box-shadow: var(--shadow-lg); opacity: 0; visibility: hidden; transform: translateY(12px); transition: opacity var(--dur-md), transform var(--dur-md), visibility var(--dur-md), background var(--dur-fast); }
.back-to-top.visible { opacity: 1; visibility: visible; transform: translateY(0); }
.back-to-top:hover { background: var(--clr-surface-2); transform: translateY(-2px); }

/* ============================================
   ENHANCED RESULT (metrics + copy) + RANDOM
   ============================================ */
.result-metrics { display: flex; flex-wrap: wrap; gap: var(--space-md); margin-top: var(--space-md); }
.result-metric { flex: 1; min-width: 120px; padding: var(--space-md); border-radius: var(--radius-md); background: var(--clr-surface-2); border: 1px solid var(--clr-border); text-align: center; }
.result-metric-value { display: block; font-family: var(--font-mono); font-size: 1.15rem; font-weight: 700; color: var(--clr-accent); }
.result-metric-label { display: block; font-size: 0.72rem; text-transform: uppercase; letter-spacing: 0.05em; color: var(--clr-text-muted); margin-top: 4px; }
.btn-copy { display: inline-flex; align-items: center; gap: 6px; margin-top: var(--space-md); padding: 8px 16px; border-radius: var(--radius-full); border: 1px solid var(--clr-border); background: var(--clr-surface); color: var(--clr-text-secondary); font-size: 0.82rem; font-weight: 600; cursor: pointer; transition: all var(--dur-fast); }
.btn-copy:hover { border-color: var(--clr-accent-light); color: var(--clr-accent); }
.btn-copy.copied { color: var(--clr-success); border-color: var(--clr-success-border); }
.btn-random { display: inline-flex; align-items: center; gap: 8px; margin: var(--space-lg) auto 0; }

@media (max-width: 600px) {
    .article-main { padding-top: calc(64px + var(--space-2xl)); }
    .article-cover { height: 220px; font-size: 3.5rem; }
    .back-to-top { bottom: 16px; right: 16px; }
}


/* ============================================
   LOCKED D FIELD — Always the unknown
   ============================================ */
.input-wrapper-locked {
    position: relative;
}

.calc-input-locked {
    opacity: 0.7;
    cursor: not-allowed !important;
    background: var(--clr-surface-3) !important;
    border-color: var(--clr-accent-light) !important;
    border-style: dashed !important;
    color: var(--clr-accent) !important;
    font-style: italic;
}

.calc-input-locked.solved {
    opacity: 1;
    border-style: solid !important;
    border-color: var(--clr-success) !important;
    background: var(--clr-success-bg) !important;
    color: var(--clr-text) !important;
    font-style: normal;
}

.locked-badge {
    position: absolute;
    bottom: -10px;
    left: 50%;
    transform: translateX(-50%);
    display: inline-flex;
    align-items: center;
    gap: 4px;
    padding: 2px 10px;
    background: var(--clr-accent-soft);
    border: 1px solid rgba(99, 102, 241, 0.15);
    border-radius: var(--radius-full);
    font-size: 0.65rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.06em;
    color: var(--clr-accent);
    white-space: nowrap;
    z-index: 2;
}

.locked-badge svg {
    flex-shrink: 0;
}


/* ============================================
   SOLUTION EXPLANATION STEPS
   ============================================ */
.solution-explanation {
    margin-top: var(--space-md);
    padding: var(--space-lg);
    background: rgba(22, 163, 74, 0.04);
    border: 1px solid rgba(22, 163, 74, 0.1);
    border-radius: var(--radius-md);
}

.solution-title {
    display: flex;
    align-items: center;
    gap: 8px;
    font-size: 0.9rem;
    font-weight: 700;
    color: var(--clr-text);
    margin-bottom: var(--space-lg);
    padding-bottom: var(--space-sm);
    border-bottom: 1px solid rgba(22, 163, 74, 0.1);
}

.solution-title svg {
    color: var(--clr-accent);
    flex-shrink: 0;
}

.solution-step {
    display: flex;
    align-items: flex-start;
    gap: 12px;
    padding: 10px 0;
    position: relative;
}

.solution-step + .solution-step {
    border-top: 1px solid rgba(22, 163, 74, 0.06);
}

.solution-step-num {
    width: 28px;
    height: 28px;
    display: flex;
    align-items: center;
    justify-content: center;
    flex-shrink: 0;
    font-family: var(--font-mono);
    font-size: 0.75rem;
    font-weight: 700;
    background: var(--clr-accent-soft);
    color: var(--clr-accent);
    border-radius: 50%;
    border: 1px solid rgba(99, 102, 241, 0.12);
}

.solution-step-final .solution-step-num {
    background: var(--clr-success-bg);
    color: var(--clr-success);
    border-color: var(--clr-success-border);
    font-size: 0.85rem;
}

.solution-step-content {
    flex: 1;
    min-width: 0;
}

.solution-step-label {
    font-size: 0.78rem;
    font-weight: 600;
    color: var(--clr-text-muted);
    margin-bottom: 2px;
}

.solution-step-math {
    font-family: var(--font-mono);
    font-size: 1rem;
    font-weight: 600;
    color: var(--clr-text);
}

.solution-step-final .solution-step-math {
    font-size: 1.15rem;
    color: var(--clr-success);
}

.solution-step-final .solution-step-math strong {
    font-size: 1.3rem;
}

.solution-verify {
    margin-top: var(--space-md);
    padding: var(--space-sm) var(--space-md);
    background: var(--clr-success-bg);
    border: 1px solid var(--clr-success-border);
    border-radius: var(--radius-sm);
    font-size: 0.82rem;
    color: var(--clr-text-secondary);
    font-family: var(--font-mono);
}

.solution-verify strong {
    color: var(--clr-success);
    font-weight: 700;
}

@media (max-width: 480px) {
    .solution-step-math {
        font-size: 0.88rem;
    }
    .solution-step-final .solution-step-math {
        font-size: 1rem;
    }
    .solution-explanation {
        padding: var(--space-md);
    }
}