/* ============================================================================
   Class 7 Interactive Learning Portal - Production CSS Stylesheet
   ============================================================================
   Mobile-first responsive design with comprehensive component styling
   ========================================================================== */

/* ============================================================================
   1. ROOT & CUSTOM PROPERTIES (CSS Variables)
   ========================================================================== */

:root {
  /* Color Palettes by Subject */
  --science-base: #0d9488;
  --science-light: #ccfbf1;
  --science-dark: #115e59;

  --math-base: #7c3aed;
  --math-light: #ede9fe;
  --math-dark: #4c1d95;

  --studies-base: #2563eb;
  --studies-light: #dbeafe;
  --studies-dark: #1e3a8a;

  --english-base: #db2777;
  --english-light: #fce7f3;
  --english-dark: #831843;

  /* Neutral Colors */
  --gray-50: #f9fafb;
  --gray-100: #f3f4f6;
  --gray-200: #e5e7eb;
  --gray-300: #d1d5db;
  --gray-400: #9ca3af;
  --gray-500: #6b7280;
  --gray-600: #4b5563;
  --gray-700: #374151;
  --gray-800: #1f2937;
  --gray-900: #111827;

  /* Semantic Colors */
  --success: #10b981;
  --warning: #f59e0b;
  --error: #ef4444;
  --info: #3b82f6;

  /* Typography */
  --font-family-body: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  --font-family-heading: 'Bricolage Grotesque', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;

  /* Font Sizes (mobile-first scaling) */
  --text-xs: 0.75rem;     /* 12px */
  --text-sm: 0.875rem;    /* 14px */
  --text-base: 1rem;      /* 16px */
  --text-lg: 1.125rem;    /* 18px */
  --text-xl: 1.25rem;     /* 20px */
  --text-2xl: 1.5rem;     /* 24px */
  --text-3xl: 1.875rem;   /* 30px */
  --text-4xl: 2.25rem;    /* 36px */
  --text-5xl: 3rem;       /* 48px */

  /* Line Heights */
  --lh-tight: 1.25;
  --lh-snug: 1.375;
  --lh-normal: 1.5;
  --lh-relaxed: 1.625;
  --lh-loose: 2;

  /* Spacing Scale (8px base) */
  --space-0: 0;
  --space-1: 0.25rem;   /* 4px */
  --space-2: 0.5rem;    /* 8px */
  --space-3: 0.75rem;   /* 12px */
  --space-4: 1rem;      /* 16px */
  --space-5: 1.25rem;   /* 20px */
  --space-6: 1.5rem;    /* 24px */
  --space-8: 2rem;      /* 32px */
  --space-10: 2.5rem;   /* 40px */
  --space-12: 3rem;     /* 48px */
  --space-16: 4rem;     /* 64px */
  --space-20: 5rem;     /* 80px */
  --space-24: 6rem;     /* 96px */

  /* Border Radius */
  --radius-sm: 0.375rem;   /* 6px */
  --radius-md: 0.5rem;     /* 8px */
  --radius-lg: 0.75rem;    /* 12px */
  --radius-xl: 1rem;       /* 16px */
  --radius-2xl: 1.5rem;    /* 24px */
  --radius-full: 9999px;

  /* Shadows */
  --shadow-xs: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
  --shadow-sm: 0 1px 3px 0 rgba(0, 0, 0, 0.1), 0 1px 2px 0 rgba(0, 0, 0, 0.06);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
  --shadow-xl: 0 20px 25px -5px rgba(0, 0, 0, 0.1), 0 10px 10px -5px rgba(0, 0, 0, 0.04);
  --shadow-2xl: 0 25px 50px -12px rgba(0, 0, 0, 0.25);

  /* Transitions */
  --duration-75: 75ms;
  --duration-100: 100ms;
  --duration-150: 150ms;
  --duration-200: 200ms;
  --duration-300: 300ms;
  --duration-500: 500ms;
  --easing-in: cubic-bezier(0.4, 0, 1, 1);
  --easing-out: cubic-bezier(0, 0, 0.2, 1);
  --easing-in-out: cubic-bezier(0.4, 0, 0.2, 1);

  /* Active Subject (default: science) */
  --subject: science;
  --subject-base: var(--science-base);
  --subject-light: var(--science-light);
  --subject-dark: var(--science-dark);

  /* Scrollbar Track */
  --scrollbar-track: var(--gray-100);
  --scrollbar-thumb: var(--gray-300);
  --scrollbar-thumb-hover: var(--gray-400);
}

/* Subject-specific root variables */
:root[data-subject="science"],
.subject-switcher-link[data-subject="science"]:has(~ .active[data-subject="science"]) {
  --subject-base: var(--science-base);
  --subject-light: var(--science-light);
  --subject-dark: var(--science-dark);
}

:root[data-subject="math"],
.subject-switcher-link[data-subject="math"]:has(~ .active[data-subject="math"]) {
  --subject-base: var(--math-base);
  --subject-light: var(--math-light);
  --subject-dark: var(--math-dark);
}

:root[data-subject="studies"],
.subject-switcher-link[data-subject="studies"]:has(~ .active[data-subject="studies"]) {
  --subject-base: var(--studies-base);
  --subject-light: var(--studies-light);
  --subject-dark: var(--studies-dark);
}

:root[data-subject="english"],
.subject-switcher-link[data-subject="english"]:has(~ .active[data-subject="english"]) {
  --subject-base: var(--english-base);
  --subject-light: var(--english-light);
  --subject-dark: var(--english-dark);
}

/* ============================================================================
   2. RESET & BASE STYLES
   ========================================================================== */

*,
*::before,
*::after {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

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

body {
  font-family: var(--font-family-body);
  font-size: var(--text-base);
  line-height: var(--lh-normal);
  color: var(--gray-900);
  background-color: var(--gray-50);
  overflow-x: hidden;
}

/* Scrollbar Styling */
::-webkit-scrollbar {
  width: 10px;
  height: 10px;
}

::-webkit-scrollbar-track {
  background: var(--scrollbar-track);
}

::-webkit-scrollbar-thumb {
  background: var(--scrollbar-thumb);
  border-radius: var(--radius-full);
  transition: background-color var(--duration-200) var(--easing-out);
}

::-webkit-scrollbar-thumb:hover {
  background: var(--scrollbar-thumb-hover);
}

/* Firefox scrollbar */
* {
  scrollbar-color: var(--scrollbar-thumb) var(--scrollbar-track);
  scrollbar-width: thin;
}

/* ============================================================================
   3. TYPOGRAPHY
   ========================================================================== */

h1, h2, h3, h4, h5, h6 {
  font-family: var(--font-family-heading);
  font-weight: 700;
  line-height: var(--lh-tight);
  color: var(--gray-900);
}

h1 {
  font-size: var(--text-4xl);
  letter-spacing: -0.02em;
}

h2 {
  font-size: var(--text-3xl);
  letter-spacing: -0.015em;
  margin-top: var(--space-12);
  margin-bottom: var(--space-4);
}

h3 {
  font-size: var(--text-2xl);
  margin-top: var(--space-8);
  margin-bottom: var(--space-3);
}

h4 {
  font-size: var(--text-xl);
  margin-top: var(--space-6);
  margin-bottom: var(--space-2);
}

h5 {
  font-size: var(--text-lg);
}

h6 {
  font-size: var(--text-base);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--gray-600);
}

p {
  margin-bottom: var(--space-4);
  line-height: var(--lh-relaxed);
}

a {
  color: var(--subject-base);
  text-decoration: none;
  transition: color var(--duration-200) var(--easing-out);
}

a:hover {
  color: var(--subject-dark);
  text-decoration: underline;
}

a:focus-visible {
  outline: 2px solid var(--subject-base);
  outline-offset: 2px;
  border-radius: var(--radius-sm);
}

strong {
  font-weight: 600;
}

em {
  font-style: italic;
}

code,
pre {
  font-family: 'Monaco', 'Courier New', monospace;
  background-color: var(--gray-100);
  border-radius: var(--radius-md);
}

code {
  padding: 2px 6px;
  font-size: 0.9em;
}

pre {
  padding: var(--space-4);
  overflow-x: auto;
  margin-bottom: var(--space-4);
}

pre code {
  padding: 0;
  background: none;
}

/* ============================================================================
   4. FORM ELEMENTS
   ========================================================================== */

input,
textarea,
select,
button {
  font-family: inherit;
  font-size: inherit;
}

input[type="text"],
input[type="email"],
input[type="number"],
input[type="password"],
textarea,
select {
  width: 100%;
  padding: var(--space-3) var(--space-4);
  border: 1px solid var(--gray-300);
  border-radius: var(--radius-md);
  background-color: white;
  transition: border-color var(--duration-200) var(--easing-out),
              box-shadow var(--duration-200) var(--easing-out);
}

input[type="text"]:focus,
input[type="email"]:focus,
input[type="number"]:focus,
input[type="password"]:focus,
textarea:focus,
select:focus {
  outline: none;
  border-color: var(--subject-base);
  box-shadow: 0 0 0 3px rgba(var(--subject-base), 0.1);
}

textarea {
  resize: vertical;
  min-height: 120px;
}

button,
input[type="button"],
input[type="submit"],
input[type="reset"] {
  cursor: pointer;
  border: none;
  font-weight: 600;
  transition: all var(--duration-200) var(--easing-out);
}

button:focus-visible,
input[type="button"]:focus-visible,
input[type="submit"]:focus-visible,
input[type="reset"]:focus-visible {
  outline: 2px solid var(--subject-base);
  outline-offset: 2px;
}

/* ============================================================================
   5. CONTAINER & LAYOUT UTILITIES
   ========================================================================== */

.container {
  width: 100%;
  max-width: 1280px;
  margin-left: auto;
  margin-right: auto;
  padding-left: var(--space-4);
  padding-right: var(--space-4);
}

@media (min-width: 768px) {
  .container {
    padding-left: var(--space-6);
    padding-right: var(--space-6);
  }
}

@media (min-width: 1024px) {
  .container {
    padding-left: var(--space-8);
    padding-right: var(--space-8);
  }
}

/* Flex utilities */
.flex {
  display: flex;
}

.flex-col {
  flex-direction: column;
}

.flex-center {
  display: flex;
  align-items: center;
  justify-content: center;
}

.gap-1 { gap: var(--space-1); }
.gap-2 { gap: var(--space-2); }
.gap-3 { gap: var(--space-3); }
.gap-4 { gap: var(--space-4); }
.gap-6 { gap: var(--space-6); }
.gap-8 { gap: var(--space-8); }

/* Grid utilities */
.grid {
  display: grid;
}

/* Spacing utilities */
.mt-4 { margin-top: var(--space-4); }
.mb-4 { margin-bottom: var(--space-4); }
.my-4 { margin-top: var(--space-4); margin-bottom: var(--space-4); }

/* ============================================================================
   6. SUBJECT SWITCHER (Sticky Top Bar)
   ========================================================================== */

.subject-switcher {
  position: sticky;
  top: 0;
  z-index: 100;
  background: white;
  border-bottom: 1px solid var(--gray-200);
  backdrop-filter: blur(10px);
  background-color: rgba(255, 255, 255, 0.95);
  box-shadow: var(--shadow-sm);
}

.switcher-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-4);
}

.switcher-brand {
  font-family: var(--font-family-heading);
  font-size: var(--text-lg);
  font-weight: 700;
  color: var(--gray-900);
  white-space: nowrap;
  margin-right: var(--space-6);
}

.switcher-brand strong {
  color: var(--subject-base);
}

.switcher-links {
  display: flex;
  gap: var(--space-2);
  flex-wrap: wrap;
  flex: 1;
}

.switcher-link {
  padding: var(--space-2) var(--space-4);
  border-radius: var(--radius-full);
  font-size: var(--text-sm);
  font-weight: 600;
  transition: all var(--duration-200) var(--easing-out);
  white-space: nowrap;
  border: 1px solid var(--gray-300);
  color: var(--gray-700);
  background-color: transparent;
  cursor: pointer;
}

.switcher-link[data-subject] {
  cursor: pointer;
}

.switcher-link[data-subject="science"] {
  --subject-color: var(--science-base);
}

.switcher-link[data-subject="math"] {
  --subject-color: var(--math-base);
}

.switcher-link[data-subject="studies"] {
  --subject-color: var(--studies-base);
}

.switcher-link[data-subject="english"] {
  --subject-color: var(--english-base);
}

.switcher-link[data-subject]:hover {
  border-color: var(--subject-color);
  color: var(--subject-color);
  background-color: rgba(var(--subject-color), 0.05);
}

.switcher-link[data-subject].active {
  border-color: var(--subject-color);
  color: white;
  background-color: var(--subject-color);
}

.switcher-link[data-subject]:focus-visible {
  outline: 2px solid var(--subject-color);
  outline-offset: 2px;
}

@media (min-width: 768px) {
  .switcher-inner {
    padding: var(--space-5);
  }

  .switcher-link {
    padding: var(--space-2) var(--space-5);
    font-size: var(--text-base);
  }
}

/* ============================================================================
   7. HERO SECTION (Landing Page)
   ========================================================================== */

.hero {
  position: relative;
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  overflow: hidden;
  background: linear-gradient(135deg, var(--gray-900) 0%, var(--gray-800) 100%);
}

.hero::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background: radial-gradient(circle at 20% 50%, rgba(13, 148, 136, 0.3) 0%, transparent 50%),
              radial-gradient(circle at 80% 80%, rgba(124, 58, 237, 0.3) 0%, transparent 50%),
              radial-gradient(circle at 40% 0%, rgba(37, 99, 235, 0.2) 0%, transparent 50%);
  pointer-events: none;
}

.hero-content {
  position: relative;
  z-index: 2;
  text-align: center;
  padding: var(--space-8);
  max-width: 800px;
  animation: fadeInUp 0.8s ease-out;
}

.hero-badge {
  display: inline-block;
  padding: var(--space-2) var(--space-4);
  background-color: rgba(var(--subject-base), 0.15);
  color: var(--subject-base);
  border-radius: var(--radius-full);
  font-size: var(--text-sm);
  font-weight: 600;
  margin-bottom: var(--space-6);
  border: 1px solid rgba(var(--subject-base), 0.3);
}

.hero h1 {
  color: white;
  font-size: var(--text-4xl);
  line-height: var(--lh-tight);
  margin-bottom: var(--space-4);
  background: linear-gradient(135deg, white 0%, rgba(255, 255, 255, 0.8) 100%);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.hero p {
  color: rgba(255, 255, 255, 0.8);
  font-size: var(--text-xl);
  margin-bottom: var(--space-8);
  line-height: var(--lh-relaxed);
}

.hero-cta {
  display: flex;
  gap: var(--space-4);
  justify-content: center;
  flex-wrap: wrap;
}

.hero-btn {
  padding: var(--space-4) var(--space-8);
  border-radius: var(--radius-lg);
  font-size: var(--text-base);
  font-weight: 700;
  transition: all var(--duration-300) var(--easing-out);
  text-transform: none;
  border: 2px solid transparent;
}

.hero-btn-primary {
  background: linear-gradient(135deg, var(--subject-base), var(--subject-dark));
  color: white;
  box-shadow: var(--shadow-lg);
}

.hero-btn-primary:hover {
  transform: translateY(-2px);
  box-shadow: var(--shadow-xl);
}

.hero-btn-secondary {
  background: transparent;
  color: white;
  border-color: white;
}

.hero-btn-secondary:hover {
  background: rgba(255, 255, 255, 0.1);
  transform: translateY(-2px);
}

@media (min-width: 768px) {
  .hero {
    min-height: 120vh;
  }

  .hero h1 {
    font-size: var(--text-5xl);
  }

  .hero p {
    font-size: var(--text-2xl);
  }
}

/* ============================================================================
   8. SUBJECT CARDS GRID
   ========================================================================== */

.subjects-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: var(--space-6);
  padding: var(--space-8) var(--space-4);
}

@media (min-width: 640px) {
  .subjects-grid {
    grid-template-columns: repeat(2, 1fr);
    padding: var(--space-8) var(--space-6);
  }
}

@media (min-width: 1024px) {
  .subjects-grid {
    grid-template-columns: repeat(4, 1fr);
    padding: var(--space-12) var(--space-8);
  }
}

.subject-card {
  background: white;
  border-radius: var(--radius-xl);
  overflow: hidden;
  box-shadow: var(--shadow-sm);
  transition: all var(--duration-300) var(--easing-out);
  display: flex;
  flex-direction: column;
  position: relative;
  border-top: 4px solid;
  cursor: pointer;
}

.subject-card[data-subject="science"] {
  border-top-color: var(--science-base);
}

.subject-card[data-subject="math"] {
  border-top-color: var(--math-base);
}

.subject-card[data-subject="studies"] {
  border-top-color: var(--studies-base);
}

.subject-card[data-subject="english"] {
  border-top-color: var(--english-base);
}

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

.subject-card-header {
  padding: var(--space-6);
  background: linear-gradient(135deg, rgba(var(--subject-base), 0.05), rgba(var(--subject-base), 0.02));
  border-bottom: 1px solid var(--gray-100);
}

.subject-card-icon {
  font-size: 2.5rem;
  margin-bottom: var(--space-2);
}

.subject-card-title {
  font-size: var(--text-xl);
  font-weight: 700;
  color: var(--gray-900);
  margin-bottom: var(--space-2);
}

.subject-card-subtitle {
  font-size: var(--text-sm);
  color: var(--gray-600);
}

.subject-card-chapters {
  padding: var(--space-6);
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
}

.chapter-item {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  padding: var(--space-2) 0;
  font-size: var(--text-sm);
  color: var(--gray-700);
  transition: color var(--duration-200) var(--easing-out);
}

.chapter-item:hover {
  color: var(--subject-base);
}

.chapter-item::before {
  content: '▪';
  color: var(--subject-base);
  font-size: 1.25rem;
}

/* ============================================================================
   9. SUBJECT LANDING PAGES
   ========================================================================== */

.subject-hero {
  position: relative;
  min-height: 60vh;
  display: flex;
  align-items: center;
  justify-content: center;
  overflow: hidden;
  background: linear-gradient(135deg, var(--subject-base), var(--subject-dark));
  padding: var(--space-8) var(--space-4);
}

.subject-hero::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background: radial-gradient(circle at 20% 50%, rgba(255, 255, 255, 0.1) 0%, transparent 50%),
              radial-gradient(circle at 80% 80%, rgba(0, 0, 0, 0.1) 0%, transparent 50%);
  pointer-events: none;
}

.subject-hero-content {
  position: relative;
  z-index: 2;
  text-align: center;
  max-width: 700px;
}

.subject-hero h1 {
  color: white;
  font-size: var(--text-4xl);
  margin-bottom: var(--space-4);
}

.subject-hero p {
  color: rgba(255, 255, 255, 0.9);
  font-size: var(--text-lg);
}

.subject-chapters-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: var(--space-6);
  padding: var(--space-8) var(--space-4);
}

@media (min-width: 640px) {
  .subject-chapters-grid {
    grid-template-columns: repeat(2, 1fr);
    padding: var(--space-8) var(--space-6);
  }
}

@media (min-width: 1024px) {
  .subject-chapters-grid {
    grid-template-columns: repeat(3, 1fr);
    padding: var(--space-12) var(--space-8);
  }
}

.ch-card {
  background: white;
  border-radius: var(--radius-lg);
  overflow: hidden;
  box-shadow: var(--shadow-sm);
  transition: all var(--duration-300) var(--easing-out);
  display: flex;
  flex-direction: column;
  cursor: pointer;
  border-left: 4px solid var(--subject-base);
}

.ch-card:hover {
  transform: translateY(-2px);
  box-shadow: var(--shadow-md);
}

.ch-card-number {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 40px;
  height: 40px;
  background: linear-gradient(135deg, var(--subject-base), var(--subject-dark));
  color: white;
  border-radius: var(--radius-md);
  font-weight: 700;
  margin: var(--space-4);
}

.ch-card-title {
  padding: 0 var(--space-4) var(--space-4) var(--space-4);
  font-size: var(--text-lg);
  font-weight: 700;
  color: var(--gray-900);
}

.ch-card-description {
  padding: 0 var(--space-4) var(--space-4) var(--space-4);
  font-size: var(--text-sm);
  color: var(--gray-600);
  flex: 1;
}

.ch-card-footer {
  padding: var(--space-4);
  border-top: 1px solid var(--gray-100);
  color: var(--subject-base);
  font-size: var(--text-sm);
  font-weight: 600;
  display: flex;
  align-items: center;
  gap: var(--space-2);
}

/* ============================================================================
   10. LESSON PAGE LAYOUT
   ========================================================================== */

.lesson {
  max-width: 800px;
  margin: var(--space-8) auto;
  padding: 0 var(--space-4);
}

@media (min-width: 768px) {
  .lesson {
    margin: var(--space-12) auto;
    padding: 0 var(--space-6);
  }
}

/* Reading progress bar */
.reading-progress {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: 3px;
  background: linear-gradient(90deg, var(--subject-base), var(--subject-dark));
  transform-origin: left;
  z-index: 200;
  animation: scaleX 0.2s ease-out;
}

@keyframes scaleX {
  from {
    transform: scaleX(0);
  }
}

/* ============================================================================
   11. HOOK BOX (Everyday Mystery)
   ========================================================================== */

.hook-box {
  background: linear-gradient(135deg, rgba(245, 158, 11, 0.1), rgba(217, 119, 6, 0.05));
  border-left: 4px solid #f59e0b;
  border-radius: var(--radius-lg);
  padding: var(--space-6);
  margin: var(--space-8) 0;
}

.hook-box-label {
  display: inline-block;
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: #92400e;
  margin-bottom: var(--space-2);
}

.hook-box h3 {
  color: #92400e;
  margin-top: 0;
}

.hook-box p {
  color: var(--gray-800);
}

/* ============================================================================
   12. FEYNMAN BRIDGE (Analogy Section)
   ========================================================================== */

.feynman-bridge {
  border: 2px solid var(--subject-base);
  border-radius: var(--radius-lg);
  padding: var(--space-6);
  margin: var(--space-8) 0;
  background: rgba(var(--subject-base), 0.02);
}

.feynman-bridge-title {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  font-size: var(--text-lg);
  font-weight: 700;
  color: var(--subject-base);
  margin-bottom: var(--space-4);
}

.feynman-bridge-title::before {
  content: '🔗';
  font-size: 1.5rem;
}

.feynman-bridge p {
  color: var(--gray-800);
}

/* ============================================================================
   13. LOGIC LADDER (Vertical Stepper)
   ========================================================================== */

.logic-ladder {
  margin: var(--space-8) 0;
  position: relative;
  padding-left: var(--space-8);
}

.logic-ladder::before {
  content: '';
  position: absolute;
  left: 15px;
  top: 30px;
  bottom: 0;
  width: 2px;
  background: var(--subject-base);
}

.ladder-step {
  margin-bottom: var(--space-8);
  position: relative;
  display: flex;
  gap: var(--space-4);
}

.ladder-step::before {
  content: attr(data-step);
  position: absolute;
  left: -34px;
  top: -5px;
  width: 30px;
  height: 30px;
  background: var(--subject-base);
  color: white;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  font-weight: 700;
  font-size: var(--text-sm);
}

.ladder-step-content h4 {
  color: var(--subject-base);
  margin-top: 0;
}

.ladder-step-content p {
  color: var(--gray-800);
  margin-bottom: 0;
}

/* ============================================================================
   14. DEEP DIVE (Blue-toned Callout)
   ========================================================================== */

.deep-dive {
  background: linear-gradient(135deg, rgba(59, 130, 246, 0.1), rgba(37, 99, 235, 0.05));
  border-left: 4px solid #3b82f6;
  border-radius: var(--radius-lg);
  padding: var(--space-6);
  margin: var(--space-8) 0;
}

.deep-dive-label {
  display: inline-block;
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: #1e40af;
  margin-bottom: var(--space-2);
}

.deep-dive h3 {
  color: #1e40af;
  margin-top: 0;
}

.deep-dive p {
  color: var(--gray-800);
}

/* ============================================================================
   15. SOCRATIC SANDBOX (Quiz Section)
   ========================================================================== */

.socratic-sandbox {
  background: white;
  border-radius: var(--radius-lg);
  border: 1px solid var(--gray-200);
  padding: var(--space-6);
  margin: var(--space-8) 0;
  box-shadow: var(--shadow-sm);
}

.socratic-sandbox-title {
  font-size: var(--text-xl);
  font-weight: 700;
  color: var(--gray-900);
  display: flex;
  align-items: center;
  gap: var(--space-2);
  margin-bottom: var(--space-6);
}

.socratic-sandbox-title::before {
  content: '❓';
  font-size: 1.5rem;
}

.quiz-levels {
  display: flex;
  flex-direction: column;
  gap: var(--space-6);
}

.quiz-level {
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
}

.quiz-level-title {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  font-weight: 700;
  color: var(--subject-base);
  padding: var(--space-3) var(--space-4);
  background-color: rgba(var(--subject-base), 0.05);
  border-radius: var(--radius-md);
}

.quiz-level-title::before {
  content: '●';
  font-size: 1.5rem;
}

.quiz-question {
  padding: var(--space-4);
  background-color: var(--gray-50);
  border-left: 3px solid var(--subject-base);
  border-radius: var(--radius-md);
}

.quiz-question p:last-child {
  margin-bottom: 0;
}

details {
  margin-top: var(--space-3);
}

summary {
  cursor: pointer;
  padding: var(--space-3) var(--space-4);
  background-color: rgba(var(--subject-base), 0.05);
  border-radius: var(--radius-md);
  font-weight: 600;
  color: var(--subject-base);
  transition: all var(--duration-200) var(--easing-out);
  user-select: none;
}

summary:hover {
  background-color: rgba(var(--subject-base), 0.1);
}

summary::marker {
  color: var(--subject-base);
}

details[open] summary {
  background-color: rgba(var(--subject-base), 0.1);
  margin-bottom: var(--space-3);
}

details-content {
  padding: var(--space-4);
  color: var(--gray-800);
}

/* ============================================================================
   16. SUBJECT-SPECIFIC COMPONENTS
   ========================================================================== */

/* Error Trap (Math) */
.error-trap {
  background: linear-gradient(135deg, rgba(239, 68, 68, 0.1), rgba(220, 38, 38, 0.05));
  border-left: 4px solid #ef4444;
  border-radius: var(--radius-lg);
  padding: var(--space-6);
  margin: var(--space-8) 0;
}

.error-trap-label {
  display: inline-block;
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: #7f1d1d;
  margin-bottom: var(--space-2);
}

.error-trap h3 {
  color: #7f1d1d;
  margin-top: 0;
}

.error-trap p {
  color: var(--gray-800);
}

/* Activity Box (Science) */
.activity-box {
  background: linear-gradient(135deg, rgba(245, 158, 11, 0.1), rgba(180, 83, 9, 0.05));
  border-left: 4px solid #f59e0b;
  border-radius: var(--radius-lg);
  padding: var(--space-6);
  margin: var(--space-8) 0;
}

.activity-box-label {
  display: inline-block;
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: #92400e;
  margin-bottom: var(--space-2);
}

.activity-box h3 {
  color: #92400e;
  margin-top: 0;
}

.activity-box p {
  color: var(--gray-800);
}

/* Roleplay Box (Social Studies) */
.roleplay-box {
  background: linear-gradient(135deg, rgba(124, 58, 237, 0.1), rgba(88, 28, 135, 0.05));
  border-left: 4px solid #7c3aed;
  border-radius: var(--radius-lg);
  padding: var(--space-6);
  margin: var(--space-8) 0;
}

.roleplay-box-label {
  display: inline-block;
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: #4c1d95;
  margin-bottom: var(--space-2);
}

.roleplay-box h3 {
  color: #4c1d95;
  margin-top: 0;
}

.roleplay-box p {
  color: var(--gray-800);
}

/* Character Box (English) */
.character-box {
  background: linear-gradient(135deg, rgba(219, 39, 119, 0.1), rgba(159, 18, 57, 0.05));
  border-left: 4px solid #db2777;
  border-radius: var(--radius-lg);
  padding: var(--space-6);
  margin: var(--space-8) 0;
}

.character-box-label {
  display: inline-block;
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: #831843;
  margin-bottom: var(--space-2);
}

.character-box h3 {
  color: #831843;
  margin-top: 0;
}

.character-box p {
  color: var(--gray-800);
}

/* Sentence Architect (English Grammar) */
.sentence-architect {
  background: var(--gray-50);
  border: 1px solid var(--gray-300);
  border-radius: var(--radius-lg);
  padding: var(--space-6);
  margin: var(--space-8) 0;
  font-family: 'Monaco', 'Courier New', monospace;
  font-size: var(--text-sm);
  line-height: var(--lh-relaxed);
  overflow-x: auto;
}

.sentence-architect-label {
  display: block;
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--english-dark);
  margin-bottom: var(--space-3);
}

.sentence-architect code {
  background: transparent;
  padding: 0;
  color: var(--gray-900);
}

/* ============================================================================
   17. THEME TAG
   ========================================================================== */

.theme-tag {
  display: inline-block;
  padding: var(--space-2) var(--space-3);
  background-color: rgba(var(--subject-base), 0.15);
  color: var(--subject-base);
  border-radius: var(--radius-md);
  font-size: var(--text-xs);
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

/* ============================================================================
   18. ANIMATIONS
   ========================================================================== */

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

@keyframes fadeIn {
  from {
    opacity: 0;
  }
  to {
    opacity: 1;
  }
}

@keyframes slideInLeft {
  from {
    opacity: 0;
    transform: translateX(-20px);
  }
  to {
    opacity: 1;
    transform: translateX(0);
  }
}

@keyframes slideInRight {
  from {
    opacity: 0;
    transform: translateX(20px);
  }
  to {
    opacity: 1;
    transform: translateX(0);
  }
}

@keyframes pulse {
  0%, 100% {
    opacity: 1;
  }
  50% {
    opacity: 0.5;
  }
}

.animate-in {
  animation: fadeInUp 0.6s ease-out;
}

.animate-in-delay-1 {
  animation: fadeInUp 0.6s ease-out 0.1s both;
}

.animate-in-delay-2 {
  animation: fadeInUp 0.6s ease-out 0.2s both;
}

.animate-in-delay-3 {
  animation: fadeInUp 0.6s ease-out 0.3s both;
}

/* ============================================================================
   19. FOOTER
   ========================================================================== */

.site-footer {
  background-color: var(--gray-900);
  color: var(--gray-300);
  padding: var(--space-12) var(--space-4);
  margin-top: var(--space-20);
  border-top: 1px solid var(--gray-800);
}

.footer-inner {
  max-width: 1280px;
  margin: 0 auto;
  display: grid;
  grid-template-columns: 1fr;
  gap: var(--space-8);
}

@media (min-width: 768px) {
  .footer-inner {
    grid-template-columns: repeat(3, 1fr);
  }
}

.footer-section h3 {
  color: white;
  font-size: var(--text-lg);
  margin-bottom: var(--space-4);
}

.footer-links {
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
}

.footer-links a {
  color: var(--gray-400);
  transition: color var(--duration-200) var(--easing-out);
}

.footer-links a:hover {
  color: white;
}

.footer-bottom {
  margin-top: var(--space-8);
  padding-top: var(--space-8);
  border-top: 1px solid var(--gray-800);
  text-align: center;
  font-size: var(--text-sm);
  color: var(--gray-500);
}

/* ============================================================================
   20. UTILITY CLASSES
   ========================================================================== */

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border-width: 0;
}

.hidden {
  display: none !important;
}

.invisible {
  visibility: hidden;
}

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

.text-left {
  text-align: left;
}

.text-right {
  text-align: right;
}

.font-bold {
  font-weight: 700;
}

.font-semibold {
  font-weight: 600;
}

.font-normal {
  font-weight: 400;
}

.text-muted {
  color: var(--gray-600);
}

.text-light {
  color: var(--gray-500);
}

.bg-light {
  background-color: var(--gray-50);
}

.rounded {
  border-radius: var(--radius-lg);
}

.shadow {
  box-shadow: var(--shadow-md);
}

/* ============================================================================
   21. RESPONSIVE MEDIA QUERIES
   ========================================================================== */

@media (max-width: 639px) {
  h1 {
    font-size: var(--text-3xl);
  }

  h2 {
    font-size: var(--text-2xl);
  }

  .switcher-brand {
    font-size: var(--text-base);
  }

  .hero {
    min-height: 70vh;
  }

  .hero h1 {
    font-size: var(--text-3xl);
  }

  .hero p {
    font-size: var(--text-lg);
  }
}

@media (min-width: 640px) and (max-width: 767px) {
  /* Tablet adjustments */
}

@media (min-width: 1024px) {
  /* Desktop adjustments */
}

/* ============================================================================
   22. PRINT STYLES
   ========================================================================== */

@media print {
  .subject-switcher,
  .site-footer,
  .hero-cta {
    display: none;
  }

  body {
    background-color: white;
    color: var(--gray-900);
  }

  a {
    color: var(--subject-base);
  }

  .page-break {
    page-break-after: always;
  }
}

/* ============================================================================
   23. ACCESSIBILITY & FOCUS STATES
   ========================================================================== */

:focus-visible {
  outline: 2px solid var(--subject-base);
  outline-offset: 2px;
}

/* Reduced motion support */
@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}

/* High contrast mode support */
@media (prefers-contrast: more) {
  body {
    border: 1px solid var(--gray-900);
  }

  button,
  a {
    font-weight: 700;
  }
}

/* Dark mode support (optional) */
@media (prefers-color-scheme: dark) {
  /* Can be implemented if needed */
}

/* ============================================================================
   24. EXTRA ENHANCEMENTS
   ========================================================================== */

/* Smooth transitions for all interactive elements */
button,
a,
input,
select,
textarea {
  transition: all var(--duration-200) var(--easing-out);
}

/* Image responsiveness */
img {
  max-width: 100%;
  height: auto;
  display: block;
}

/* Tables */
table {
  width: 100%;
  border-collapse: collapse;
  margin: var(--space-6) 0;
}

th,
td {
  padding: var(--space-3) var(--space-4);
  text-align: left;
  border-bottom: 1px solid var(--gray-200);
}

th {
  background-color: var(--gray-100);
  font-weight: 700;
  color: var(--gray-900);
}

/* Blockquote */
blockquote {
  border-left: 4px solid var(--subject-base);
  padding-left: var(--space-4);
  margin: var(--space-6) 0;
  font-style: italic;
  color: var(--gray-700);
}

/* Lists */
ul,
ol {
  margin: var(--space-4) 0;
  padding-left: var(--space-8);
}

li {
  margin-bottom: var(--space-2);
}

dl {
  margin: var(--space-4) 0;
}

dt {
  font-weight: 700;
  margin-top: var(--space-4);
}

dd {
  margin-left: var(--space-6);
  color: var(--gray-700);
}

/* Horizontal rule */
hr {
  border: none;
  border-top: 1px solid var(--gray-200);
  margin: var(--space-8) 0;
}

/* ============================================================================
   ADDITIONAL COMPONENTS (Chapter Pages)
   ========================================================================== */

/* --- Chapter Hero Banner --- */
.chapter-hero {
  padding: var(--space-12) var(--space-4) var(--space-8);
  text-align: center;
  color: #fff;
}
.chapter-hero.science  { background: linear-gradient(135deg, var(--science-dark), var(--science-base)); }
.chapter-hero.math     { background: linear-gradient(135deg, var(--math-dark), var(--math-base)); }
.chapter-hero.social   { background: linear-gradient(135deg, var(--studies-dark), var(--studies-base)); }
.chapter-hero.english  { background: linear-gradient(135deg, var(--english-dark), var(--english-base)); }

.chapter-hero .chapter-label {
  display: inline-block;
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  opacity: 0.8;
  margin-bottom: var(--space-2);
}
.chapter-hero h1 {
  font-size: clamp(1.75rem, 4vw, 2.5rem);
  margin-bottom: var(--space-2);
}
.chapter-hero .chapter-subtitle {
  font-size: var(--text-base);
  opacity: 0.85;
  max-width: 50ch;
  margin-inline: auto;
}

/* --- Breadcrumb --- */
.breadcrumb {
  font-size: var(--text-sm);
  color: var(--gray-500);
  margin-bottom: var(--space-8);
  padding-bottom: var(--space-4);
  border-bottom: 1px solid var(--gray-200);
}
.breadcrumb a {
  color: var(--gray-600);
  font-weight: 500;
  transition: color 0.15s ease;
}
.breadcrumb a:hover { color: var(--gray-900); text-decoration: underline; }

/* --- Hook Box Labels --- */
.hook-label {
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: #a16207;
  margin-bottom: var(--space-1);
}

/* --- Feynman Bridge Labels --- */
.bridge-label {
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--gray-500);
  margin-bottom: var(--space-2);
}

/* --- Logic Ladder Steps --- */
.logic-step {
  position: relative;
  padding: var(--space-4) 0 var(--space-4) var(--space-8);
  border-bottom: 1px solid var(--gray-100);
}
.logic-step:last-child { border-bottom: none; }

.logic-step::before {
  counter-increment: logic-counter;
  content: counter(logic-counter);
  position: absolute;
  left: 0;
  top: var(--space-4);
  width: 1.75rem;
  height: 1.75rem;
  border-radius: 50%;
  background: var(--gray-800);
  color: #fff;
  font-size: var(--text-xs);
  font-weight: 700;
  display: grid;
  place-items: center;
}

.logic-ladder {
  counter-reset: logic-counter;
  margin-bottom: var(--space-8);
}

.logic-step h4 {
  font-family: var(--font-family-heading);
  font-size: var(--text-base);
  margin-bottom: var(--space-1);
  color: var(--gray-800);
}

/* --- Deep Dive Labels --- */
.dd-label {
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--studies-dark);
  margin-bottom: var(--space-1);
}

/* --- Quiz Level Badges --- */
.level-badge {
  display: inline-block;
  font-size: var(--text-xs);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  padding: 0.15rem 0.6rem;
  border-radius: 999px;
  margin-bottom: var(--space-2);
}
.quiz-level.predict .level-badge { background: #fef9c3; color: #854d0e; }
.quiz-level.why .level-badge     { background: #dbeafe; color: #1e40af; }
.quiz-level.apply .level-badge   { background: #dcfce7; color: #166534; }

/* --- Chapter Navigation (Prev/Next) --- */
.chapter-nav {
  display: flex;
  justify-content: space-between;
  gap: var(--space-4);
  margin-top: var(--space-12);
  padding-top: var(--space-6);
  border-top: 2px solid var(--gray-200);
}
.chapter-nav a {
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: var(--space-1);
  padding: var(--space-4);
  border: 1px solid var(--gray-200);
  border-radius: var(--radius-md);
  transition: all 0.2s ease;
  text-decoration: none;
  color: inherit;
}
.chapter-nav a:hover {
  border-color: var(--gray-400);
  background: var(--gray-50);
  transform: translateY(-1px);
  box-shadow: 0 2px 8px rgba(0,0,0,0.06);
}
.chapter-nav .prev { text-align: left; }
.chapter-nav .next { text-align: right; margin-left: auto; }
.nav-label {
  font-size: var(--text-xs);
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--gray-400);
}
.nav-title {
  font-size: var(--text-sm);
  font-weight: 600;
  color: var(--gray-700);
}

@media (max-width: 640px) {
  .chapter-nav { flex-direction: column; }
  .chapter-nav .next { text-align: left; margin-left: 0; }
}

/* ============================================================================
   END OF STYLESHEET
   ========================================================================== */