/* ============================================================
   QUANTUM MECHANICS STUDY PLATFORM — DESIGN SYSTEM
   Art direction: Deep space / blackboard physics aesthetic
   Deep navy-black backgrounds, chalk-white text, quantum teal accent,
   gold for mathematical highlights
   ============================================================ */

@import url('https://fonts.googleapis.com/css2?family=Space+Grotesk:wght@300;400;500;600;700&family=Crimson+Pro:ital,wght@0,300;0,400;0,600;1,300;1,400&family=JetBrains+Mono:wght@400;500&display=swap');

/* ============================================================
   DESIGN TOKENS
   ============================================================ */
:root {
  /* Type scale */
  --text-xs:   clamp(0.75rem,  0.7rem  + 0.25vw, 0.875rem);
  --text-sm:   clamp(0.875rem, 0.8rem  + 0.35vw, 1rem);
  --text-base: clamp(1rem,     0.95rem + 0.25vw, 1.125rem);
  --text-lg:   clamp(1.125rem, 1rem    + 0.75vw, 1.5rem);
  --text-xl:   clamp(1.5rem,   1.2rem  + 1.25vw, 2.25rem);
  --text-2xl:  clamp(2rem,     1.2rem  + 2.5vw,  3.5rem);
  --text-hero: clamp(2.5rem,   1rem    + 4vw,    5rem);

  /* Spacing */
  --space-1:  0.25rem;
  --space-2:  0.5rem;
  --space-3:  0.75rem;
  --space-4:  1rem;
  --space-5:  1.25rem;
  --space-6:  1.5rem;
  --space-8:  2rem;
  --space-10: 2.5rem;
  --space-12: 3rem;
  --space-16: 4rem;
  --space-20: 5rem;
  --space-24: 6rem;

  /* Content widths */
  --content-narrow: 680px;
  --content-default: 960px;
  --content-wide: 1200px;

  /* Radii */
  --radius-sm: 0.375rem;
  --radius-md: 0.5rem;
  --radius-lg: 0.75rem;
  --radius-xl: 1.25rem;
  --radius-full: 9999px;

  /* Transitions */
  --transition: 180ms cubic-bezier(0.16, 1, 0.3, 1);

  /* Fonts */
  --font-display: 'Space Grotesk', 'Helvetica Neue', sans-serif;
  --font-body: 'Crimson Pro', Georgia, serif;
  --font-mono: 'JetBrains Mono', 'Courier New', monospace;
}

/* DARK MODE (default — physics chalkboard aesthetic) */
:root, [data-theme="dark"] {
  --color-bg:             #0d0f14;
  --color-surface:        #11141b;
  --color-surface-2:      #161a24;
  --color-surface-offset: #1c2030;
  --color-surface-3:      #202538;
  --color-divider:        #252b3f;
  --color-border:         #2d3450;

  --color-text:           #e8ecf4;
  --color-text-muted:     #8a93b2;
  --color-text-faint:     #4a5270;
  --color-text-inverse:   #0d0f14;

  /* Quantum teal — primary accent */
  --color-primary:        #00c9d7;
  --color-primary-hover:  #00a8b5;
  --color-primary-active: #008899;
  --color-primary-dim:    #003a40;

  /* Gold — math highlights */
  --color-gold:           #f0c040;
  --color-gold-dim:       #3d3010;

  /* Violet — secondary */
  --color-violet:         #9b7fe8;
  --color-violet-dim:     #2a1f4a;

  /* Success/correct */
  --color-success:        #4dd692;
  --color-success-dim:    #0e3024;

  /* Error/incorrect */
  --color-error:          #f07070;
  --color-error-dim:      #3a1010;

  /* Shadows */
  --shadow-sm: 0 1px 3px oklch(0 0 0 / 0.3);
  --shadow-md: 0 4px 16px oklch(0 0 0 / 0.4);
  --shadow-lg: 0 12px 40px oklch(0 0 0 / 0.5);
  --shadow-glow: 0 0 24px oklch(0.7 0.2 195 / 0.2);
}

/* LIGHT MODE */
[data-theme="light"] {
  --color-bg:             #f4f5f8;
  --color-surface:        #ffffff;
  --color-surface-2:      #f8f9fc;
  --color-surface-offset: #eef0f6;
  --color-surface-3:      #e8ebf2;
  --color-divider:        #dde0ec;
  --color-border:         #c8cce0;

  --color-text:           #1a1d2e;
  --color-text-muted:     #5a6080;
  --color-text-faint:     #9aa0bc;
  --color-text-inverse:   #f4f5f8;

  --color-primary:        #007a85;
  --color-primary-hover:  #005f6a;
  --color-primary-active: #004a54;
  --color-primary-dim:    #d0edf0;

  --color-gold:           #b07a00;
  --color-gold-dim:       #fdf3d0;

  --color-violet:         #5e38c4;
  --color-violet-dim:     #ede7fb;

  --color-success:        #1a7a44;
  --color-success-dim:    #d4f2e4;

  --color-error:          #c02020;
  --color-error-dim:      #fde8e8;

  --shadow-sm: 0 1px 3px oklch(0.3 0.05 250 / 0.08);
  --shadow-md: 0 4px 16px oklch(0.3 0.05 250 / 0.1);
  --shadow-lg: 0 12px 40px oklch(0.3 0.05 250 / 0.14);
  --shadow-glow: 0 0 24px oklch(0.5 0.15 195 / 0.15);
}

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

html {
  -webkit-text-size-adjust: none;
  text-rendering: optimizeLegibility;
  -webkit-font-smoothing: antialiased;
  scroll-behavior: smooth;
  scroll-padding-top: var(--space-16);
}

body {
  min-height: 100dvh;
  font-family: var(--font-body);
  font-size: var(--text-base);
  line-height: 1.7;
  color: var(--color-text);
  background-color: var(--color-bg);
  transition: background-color var(--transition), color var(--transition);
}

img, picture, video, canvas, svg { display: block; max-width: 100%; height: auto; }
ul[role="list"], ol[role="list"] { list-style: none; }
input, button, textarea, select { font: inherit; color: inherit; }
h1, h2, h3, h4, h5, h6 { text-wrap: balance; line-height: 1.2; font-family: var(--font-display); }
p, li, figcaption { text-wrap: pretty; max-width: 72ch; }
button { cursor: pointer; background: none; border: none; }
table { border-collapse: collapse; width: 100%; }

:focus-visible {
  outline: 2px solid var(--color-primary);
  outline-offset: 3px;
  border-radius: var(--radius-sm);
}

::selection {
  background: color-mix(in oklab, var(--color-primary) 25%, transparent);
  color: var(--color-text);
}

a, button, [role="button"] {
  transition: color var(--transition), background var(--transition),
              border-color var(--transition), box-shadow var(--transition),
              opacity var(--transition);
}

.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;
}

/* ============================================================
   LAYOUT UTILITIES
   ============================================================ */
.container {
  width: 100%;
  max-width: var(--content-default);
  margin-inline: auto;
  padding-inline: clamp(var(--space-4), 5vw, var(--space-12));
}
.container--wide  { max-width: var(--content-wide); }
.container--narrow { max-width: var(--content-narrow); }

/* ============================================================
   NAV
   ============================================================ */
.site-nav {
  position: sticky;
  top: 0;
  z-index: 100;
  background: color-mix(in oklab, var(--color-bg) 85%, transparent);
  backdrop-filter: blur(16px);
  -webkit-backdrop-filter: blur(16px);
  border-bottom: 1px solid var(--color-divider);
}

.nav-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-4);
  height: 60px;
}

.nav-logo {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  text-decoration: none;
  color: var(--color-text);
  font-family: var(--font-display);
  font-weight: 700;
  font-size: var(--text-sm);
  letter-spacing: -0.01em;
}

.nav-logo svg { flex-shrink: 0; }

.nav-links {
  display: flex;
  align-items: center;
  gap: var(--space-1);
  list-style: none;
}

.nav-links a {
  display: flex;
  align-items: center;
  padding: var(--space-2) var(--space-3);
  border-radius: var(--radius-md);
  text-decoration: none;
  color: var(--color-text-muted);
  font-family: var(--font-display);
  font-size: var(--text-sm);
  font-weight: 500;
  white-space: nowrap;
}

.nav-links a:hover {
  color: var(--color-text);
  background: var(--color-surface-offset);
}

.nav-links a.active {
  color: var(--color-primary);
  background: var(--color-primary-dim);
}

.nav-actions {
  display: flex;
  align-items: center;
  gap: var(--space-2);
}

.theme-toggle {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 36px;
  height: 36px;
  border-radius: var(--radius-md);
  color: var(--color-text-muted);
  background: transparent;
  border: 1px solid var(--color-border);
}

.theme-toggle:hover {
  color: var(--color-text);
  background: var(--color-surface-offset);
  border-color: var(--color-text-faint);
}

/* ============================================================
   HERO — HOME PAGE
   ============================================================ */
.hero {
  position: relative;
  overflow: hidden;
  padding-block: clamp(var(--space-16), 10vw, var(--space-24));
  text-align: center;
}

.hero-bg {
  position: absolute;
  inset: 0;
  z-index: 0;
  background: radial-gradient(ellipse 80% 60% at 50% 30%,
    color-mix(in oklab, var(--color-primary) 12%, transparent),
    transparent 70%),
    radial-gradient(ellipse 60% 50% at 80% 70%,
    color-mix(in oklab, var(--color-violet) 8%, transparent),
    transparent 70%);
  pointer-events: none;
}

.hero-content {
  position: relative;
  z-index: 1;
}

.hero-badge {
  display: inline-flex;
  align-items: center;
  gap: var(--space-2);
  padding: var(--space-1) var(--space-4);
  border-radius: var(--radius-full);
  background: var(--color-primary-dim);
  border: 1px solid color-mix(in oklab, var(--color-primary) 30%, transparent);
  font-family: var(--font-display);
  font-size: var(--text-xs);
  font-weight: 600;
  color: var(--color-primary);
  letter-spacing: 0.06em;
  text-transform: uppercase;
  margin-bottom: var(--space-6);
}

.hero-title {
  font-size: var(--text-2xl);
  font-weight: 700;
  letter-spacing: -0.03em;
  color: var(--color-text);
  margin-bottom: var(--space-6);
}

.hero-title em {
  font-style: normal;
  color: var(--color-primary);
}

.hero-subtitle {
  font-size: var(--text-lg);
  color: var(--color-text-muted);
  max-width: 56ch;
  margin-inline: auto;
  margin-bottom: var(--space-10);
  font-weight: 300;
  line-height: 1.6;
}

.hero-math {
  font-family: var(--font-mono);
  font-size: var(--text-sm);
  color: var(--color-gold);
  opacity: 0.7;
  margin-bottom: var(--space-10);
  letter-spacing: 0.02em;
}

.btn-group {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-4);
  flex-wrap: wrap;
}

/* ============================================================
   BUTTONS
   ============================================================ */
.btn {
  display: inline-flex;
  align-items: center;
  gap: var(--space-2);
  padding: var(--space-3) var(--space-6);
  border-radius: var(--radius-md);
  font-family: var(--font-display);
  font-size: var(--text-sm);
  font-weight: 600;
  text-decoration: none;
  white-space: nowrap;
  border: 1.5px solid transparent;
}

.btn-primary {
  background: var(--color-primary);
  color: var(--color-text-inverse);
  border-color: var(--color-primary);
  box-shadow: var(--shadow-glow);
}

.btn-primary:hover {
  background: var(--color-primary-hover);
  border-color: var(--color-primary-hover);
  transform: translateY(-1px);
  box-shadow: var(--shadow-lg), var(--shadow-glow);
}

.btn-secondary {
  background: transparent;
  color: var(--color-text);
  border-color: var(--color-border);
}

.btn-secondary:hover {
  background: var(--color-surface-offset);
  border-color: var(--color-text-faint);
}

/* ============================================================
   STATS BAR
   ============================================================ */
.stats-bar {
  display: flex;
  justify-content: center;
  gap: clamp(var(--space-8), 6vw, var(--space-20));
  flex-wrap: wrap;
  padding-block: var(--space-10);
  border-top: 1px solid var(--color-divider);
  border-bottom: 1px solid var(--color-divider);
  margin-bottom: var(--space-16);
}

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

.stat-number {
  display: block;
  font-size: var(--text-xl);
  font-weight: 700;
  font-family: var(--font-display);
  color: var(--color-primary);
  letter-spacing: -0.02em;
}

.stat-label {
  font-size: var(--text-xs);
  color: var(--color-text-muted);
  font-family: var(--font-display);
  text-transform: uppercase;
  letter-spacing: 0.06em;
}

/* ============================================================
   SECTION HEADERS
   ============================================================ */
.section-header {
  text-align: center;
  margin-bottom: var(--space-12);
}

.section-eyebrow {
  display: inline-block;
  font-family: var(--font-display);
  font-size: var(--text-xs);
  font-weight: 600;
  color: var(--color-primary);
  letter-spacing: 0.1em;
  text-transform: uppercase;
  margin-bottom: var(--space-3);
}

.section-title {
  font-size: var(--text-xl);
  font-weight: 700;
  letter-spacing: -0.02em;
  color: var(--color-text);
  margin-bottom: var(--space-4);
}

.section-desc {
  font-size: var(--text-base);
  color: var(--color-text-muted);
  max-width: 54ch;
  margin-inline: auto;
}

/* ============================================================
   COURSE CARDS
   ============================================================ */
.courses-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(min(100%, 300px), 1fr));
  gap: var(--space-6);
  margin-bottom: var(--space-16);
}

.course-card {
  position: relative;
  display: flex;
  flex-direction: column;
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-xl);
  overflow: hidden;
  text-decoration: none;
  color: var(--color-text);
  box-shadow: var(--shadow-sm);
  transition: transform var(--transition), box-shadow var(--transition),
              border-color var(--transition);
}

.course-card:hover {
  transform: translateY(-3px);
  box-shadow: var(--shadow-md), var(--shadow-glow);
  border-color: color-mix(in oklab, var(--color-primary) 40%, var(--color-border));
}

.card-image {
  width: 100%;
  aspect-ratio: 16 / 9;
  object-fit: cover;
  background: var(--color-surface-offset);
}

.card-image-placeholder {
  width: 100%;
  aspect-ratio: 16 / 9;
  background: linear-gradient(135deg, var(--color-surface-offset), var(--color-surface-3));
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 2.5rem;
}

.card-body {
  flex: 1;
  display: flex;
  flex-direction: column;
  padding: var(--space-5);
  gap: var(--space-3);
}

.card-meta {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  flex-wrap: wrap;
}

.tag {
  display: inline-flex;
  align-items: center;
  padding: var(--space-1) var(--space-2);
  border-radius: var(--radius-sm);
  font-family: var(--font-display);
  font-size: var(--text-xs);
  font-weight: 500;
}

.tag-primary { background: var(--color-primary-dim); color: var(--color-primary); }
.tag-gold { background: var(--color-gold-dim); color: var(--color-gold); }
.tag-violet { background: var(--color-violet-dim); color: var(--color-violet); }

.card-title {
  font-size: var(--text-base);
  font-weight: 600;
  line-height: 1.3;
  color: var(--color-text);
}

.card-desc {
  font-size: var(--text-sm);
  color: var(--color-text-muted);
  line-height: 1.6;
  flex: 1;
}

.card-footer {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-3) var(--space-5);
  border-top: 1px solid var(--color-divider);
  font-family: var(--font-display);
  font-size: var(--text-xs);
  color: var(--color-text-faint);
}

.card-footer-link {
  color: var(--color-primary);
  font-weight: 600;
  display: flex;
  align-items: center;
  gap: var(--space-1);
}

/* ============================================================
   LECTURE PAGE HEADER
   ============================================================ */
.page-section {
  padding-block: clamp(var(--space-12), 6vw, var(--space-20));
}

.lecture-hero {
  padding-block: var(--space-12) var(--space-8);
  border-bottom: 1px solid var(--color-divider);
  margin-bottom: var(--space-12);
}

.breadcrumb {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  font-family: var(--font-display);
  font-size: var(--text-xs);
  color: var(--color-text-faint);
  margin-bottom: var(--space-5);
}

.breadcrumb a { color: var(--color-text-muted); text-decoration: none; }
.breadcrumb a:hover { color: var(--color-primary); }
.breadcrumb-sep { color: var(--color-text-faint); }

.lecture-number {
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  color: var(--color-primary);
  font-weight: 500;
  letter-spacing: 0.06em;
  text-transform: uppercase;
  margin-bottom: var(--space-3);
}

.lecture-title {
  font-size: var(--text-xl);
  font-weight: 700;
  letter-spacing: -0.02em;
  color: var(--color-text);
  margin-bottom: var(--space-5);
}

.lecture-subtitle {
  font-size: var(--text-base);
  color: var(--color-text-muted);
  max-width: 62ch;
  line-height: 1.6;
  margin-bottom: var(--space-6);
}

.lecture-nav-pills {
  display: flex;
  gap: var(--space-2);
  flex-wrap: wrap;
}

.pill {
  display: inline-flex;
  align-items: center;
  gap: var(--space-2);
  padding: var(--space-2) var(--space-4);
  border-radius: var(--radius-full);
  font-family: var(--font-display);
  font-size: var(--text-xs);
  font-weight: 600;
  text-decoration: none;
  border: 1.5px solid transparent;
  white-space: nowrap;
}

.pill-outline {
  background: transparent;
  color: var(--color-text-muted);
  border-color: var(--color-border);
}

.pill-outline:hover, .pill-outline.active {
  background: var(--color-primary-dim);
  color: var(--color-primary);
  border-color: color-mix(in oklab, var(--color-primary) 40%, transparent);
}

/* ============================================================
   TWO-COLUMN LAYOUT
   ============================================================ */
.two-col {
  display: grid;
  grid-template-columns: 1fr 340px;
  gap: var(--space-10);
  align-items: start;
}

@media (max-width: 900px) {
  .two-col { grid-template-columns: 1fr; }
}

/* ============================================================
   ARTICLE / THEORY CONTENT
   ============================================================ */
.article-body {
  font-size: var(--text-base);
  line-height: 1.8;
  color: var(--color-text);
}

.article-body h2 {
  font-size: var(--text-lg);
  font-weight: 700;
  margin-top: var(--space-10);
  margin-bottom: var(--space-4);
  color: var(--color-text);
  padding-bottom: var(--space-3);
  border-bottom: 1px solid var(--color-divider);
}

.article-body h3 {
  font-size: var(--text-base);
  font-weight: 700;
  margin-top: var(--space-6);
  margin-bottom: var(--space-3);
  color: var(--color-text);
  font-family: var(--font-display);
}

.article-body p {
  margin-bottom: var(--space-4);
  max-width: 70ch;
}

.article-body ul, .article-body ol {
  margin-bottom: var(--space-4);
  padding-left: var(--space-6);
}

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

.article-body blockquote {
  border-left: 3px solid var(--color-primary);
  padding: var(--space-4) var(--space-6);
  margin-block: var(--space-6);
  background: var(--color-primary-dim);
  border-radius: 0 var(--radius-md) var(--radius-md) 0;
  font-style: italic;
  color: var(--color-text-muted);
}

.article-body blockquote strong {
  color: var(--color-text);
  font-style: normal;
}

.article-body .math-block {
  overflow-x: auto;
  padding: var(--space-4) var(--space-6);
  background: var(--color-surface-offset);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  margin-block: var(--space-5);
  text-align: center;
  font-size: 1.1em;
}

.article-body .math-block .katex-display {
  margin: 0;
  overflow-x: auto;
  overflow-y: hidden;
}

.article-body table {
  margin-block: var(--space-6);
  font-size: var(--text-sm);
  border-radius: var(--radius-md);
  overflow: hidden;
  border: 1px solid var(--color-border);
}

.article-body th {
  background: var(--color-surface-offset);
  color: var(--color-text);
  font-family: var(--font-display);
  font-weight: 600;
  padding: var(--space-3) var(--space-4);
  text-align: left;
  border-bottom: 1px solid var(--color-border);
}

.article-body td {
  padding: var(--space-3) var(--space-4);
  border-bottom: 1px solid var(--color-divider);
  color: var(--color-text-muted);
  vertical-align: top;
}

.article-body tr:last-child td { border-bottom: none; }

.key-equation {
  display: flex;
  flex-direction: column;
  align-items: center;
  padding: var(--space-5) var(--space-8);
  background: linear-gradient(135deg,
    color-mix(in oklab, var(--color-primary) 8%, var(--color-surface-2)),
    color-mix(in oklab, var(--color-gold) 5%, var(--color-surface-2)));
  border: 1px solid color-mix(in oklab, var(--color-primary) 20%, var(--color-border));
  border-radius: var(--radius-lg);
  margin-block: var(--space-6);
  gap: var(--space-3);
}

.key-equation-label {
  font-family: var(--font-display);
  font-size: var(--text-xs);
  font-weight: 600;
  color: var(--color-primary);
  text-transform: uppercase;
  letter-spacing: 0.08em;
}

.key-equation .katex-display {
  margin: 0 !important;
  overflow-x: auto;
  overflow-y: hidden;
}

/* ============================================================
   SIDEBAR / TABLE OF CONTENTS
   ============================================================ */
.sidebar {
  display: flex;
  flex-direction: column;
  gap: var(--space-6);
}

.sidebar-card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-5);
}

.sidebar-title {
  font-family: var(--font-display);
  font-size: var(--text-sm);
  font-weight: 700;
  color: var(--color-text);
  margin-bottom: var(--space-4);
  padding-bottom: var(--space-3);
  border-bottom: 1px solid var(--color-divider);
  display: flex;
  align-items: center;
  gap: var(--space-2);
}

.toc-list {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: var(--space-1);
}

.toc-list a {
  display: block;
  padding: var(--space-2) var(--space-3);
  border-radius: var(--radius-sm);
  font-size: var(--text-sm);
  text-decoration: none;
  color: var(--color-text-muted);
  line-height: 1.4;
}

.toc-list a:hover { background: var(--color-surface-offset); color: var(--color-text); }
.toc-list a.toc-h3 { padding-left: var(--space-6); font-size: var(--text-xs); }

/* Key concepts panel */
.concept-list {
  display: flex;
  flex-direction: column;
  gap: var(--space-2);
}

.concept-item {
  display: flex;
  align-items: flex-start;
  gap: var(--space-2);
  padding: var(--space-2);
  border-radius: var(--radius-sm);
  font-size: var(--text-sm);
  color: var(--color-text-muted);
}

.concept-dot {
  width: 6px;
  height: 6px;
  border-radius: 50%;
  background: var(--color-primary);
  flex-shrink: 0;
  margin-top: 0.45em;
}

/* ============================================================
   EXERCISES SECTION
   ============================================================ */
.exercises-section {
  margin-top: var(--space-12);
  padding-top: var(--space-12);
  border-top: 1px solid var(--color-divider);
}

.exercises-header {
  margin-bottom: var(--space-8);
}

.exercises-title {
  font-size: var(--text-xl);
  font-weight: 700;
  letter-spacing: -0.02em;
  color: var(--color-text);
  margin-bottom: var(--space-3);
}

.exercises-desc {
  color: var(--color-text-muted);
  font-size: var(--text-base);
}

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

.exercise-card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-xl);
  overflow: hidden;
  transition: border-color var(--transition), box-shadow var(--transition);
}

.exercise-card:hover { border-color: var(--color-text-faint); }

.exercise-header {
  display: flex;
  align-items: center;
  gap: var(--space-4);
  padding: var(--space-5) var(--space-6);
  border-bottom: 1px solid var(--color-divider);
  cursor: pointer;
  user-select: none;
}

.exercise-num {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 36px;
  height: 36px;
  border-radius: var(--radius-md);
  background: var(--color-surface-offset);
  font-family: var(--font-display);
  font-size: var(--text-sm);
  font-weight: 700;
  color: var(--color-primary);
  flex-shrink: 0;
}

.exercise-info {
  flex: 1;
}

.exercise-title {
  font-family: var(--font-display);
  font-size: var(--text-base);
  font-weight: 600;
  color: var(--color-text);
  margin-bottom: var(--space-1);
}

.exercise-tags {
  display: flex;
  gap: var(--space-2);
  flex-wrap: wrap;
}

.difficulty {
  font-family: var(--font-display);
  font-size: var(--text-xs);
  font-weight: 500;
  padding: 2px var(--space-2);
  border-radius: var(--radius-sm);
}

.difficulty-easy { background: var(--color-success-dim); color: var(--color-success); }
.difficulty-medium { background: var(--color-gold-dim); color: var(--color-gold); }
.difficulty-hard { background: var(--color-error-dim); color: var(--color-error); }

.exercise-toggle {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 32px;
  height: 32px;
  border-radius: var(--radius-md);
  color: var(--color-text-faint);
  transition: transform var(--transition), color var(--transition);
  flex-shrink: 0;
}

.exercise-card.open .exercise-toggle {
  transform: rotate(180deg);
  color: var(--color-primary);
}

.exercise-body {
  padding: var(--space-6);
  border-bottom: 1px solid var(--color-divider);
}

.exercise-body p {
  color: var(--color-text);
  line-height: 1.7;
  margin-bottom: var(--space-3);
}

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

.exercise-math {
  background: var(--color-surface-offset);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: var(--space-4) var(--space-5);
  margin-block: var(--space-4);
  text-align: center;
  overflow-x: auto;
}

.exercise-body ul, .exercise-body ol {
  padding-left: var(--space-6);
  margin-bottom: var(--space-4);
}

.exercise-body li { margin-bottom: var(--space-2); color: var(--color-text); }

.solution-area {
  display: none;
  padding: var(--space-6);
  background: color-mix(in oklab, var(--color-success) 5%, var(--color-bg));
  border-top: 1px solid color-mix(in oklab, var(--color-success) 20%, var(--color-border));
}

.exercise-card.open .solution-area { display: block; }

.solution-header {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  margin-bottom: var(--space-5);
  font-family: var(--font-display);
  font-size: var(--text-sm);
  font-weight: 700;
  color: var(--color-success);
}

.solution-header svg { flex-shrink: 0; }

.solution-step {
  display: flex;
  gap: var(--space-4);
  margin-bottom: var(--space-5);
  padding-bottom: var(--space-5);
  border-bottom: 1px solid var(--color-divider);
}

.solution-step:last-child {
  margin-bottom: 0;
  padding-bottom: 0;
  border-bottom: none;
}

.step-label {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 28px;
  height: 28px;
  border-radius: var(--radius-full);
  background: var(--color-success-dim);
  font-family: var(--font-display);
  font-size: var(--text-xs);
  font-weight: 700;
  color: var(--color-success);
  flex-shrink: 0;
  margin-top: 2px;
}

.step-content {
  flex: 1;
}

.step-content p {
  color: var(--color-text);
  margin-bottom: var(--space-3);
  line-height: 1.7;
}

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

.step-math {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: var(--space-4);
  margin-block: var(--space-3);
  text-align: center;
  overflow-x: auto;
}

.solution-box {
  padding: var(--space-4) var(--space-5);
  background: color-mix(in oklab, var(--color-success) 10%, var(--color-surface));
  border: 2px solid color-mix(in oklab, var(--color-success) 30%, transparent);
  border-radius: var(--radius-md);
  margin-top: var(--space-4);
  text-align: center;
}

.solution-box-label {
  font-family: var(--font-display);
  font-size: var(--text-xs);
  font-weight: 700;
  color: var(--color-success);
  text-transform: uppercase;
  letter-spacing: 0.06em;
  margin-bottom: var(--space-2);
}

.show-solution-btn {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  width: 100%;
  padding: var(--space-3) var(--space-5);
  background: transparent;
  border: none;
  border-top: 1px solid var(--color-divider);
  font-family: var(--font-display);
  font-size: var(--text-sm);
  font-weight: 600;
  color: var(--color-text-muted);
  cursor: pointer;
}

.show-solution-btn:hover { color: var(--color-primary); background: var(--color-primary-dim); }

/* ============================================================
   FORMULA REFERENCE CARD
   ============================================================ */
.formula-grid {
  display: grid;
  gap: var(--space-3);
}

.formula-item {
  padding: var(--space-3) var(--space-4);
  background: var(--color-surface-offset);
  border-radius: var(--radius-md);
  border-left: 3px solid var(--color-gold);
}

.formula-label {
  font-family: var(--font-display);
  font-size: var(--text-xs);
  color: var(--color-text-muted);
  margin-bottom: var(--space-2);
  font-weight: 500;
}

.formula-math {
  font-family: var(--font-mono);
  font-size: var(--text-sm);
  color: var(--color-gold);
  overflow-x: auto;
}

/* ============================================================
   LECTURE NAVIGATION (prev/next)
   ============================================================ */
.lecture-nav {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-4);
  margin-top: var(--space-12);
  padding-top: var(--space-8);
  border-top: 1px solid var(--color-divider);
}

.lecture-nav-btn {
  display: flex;
  flex-direction: column;
  gap: var(--space-1);
  padding: var(--space-4) var(--space-5);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  text-decoration: none;
  color: var(--color-text);
  transition: border-color var(--transition), transform var(--transition), box-shadow var(--transition);
}

.lecture-nav-btn:hover {
  border-color: var(--color-primary);
  transform: translateY(-2px);
  box-shadow: var(--shadow-md);
}

.lecture-nav-btn.next { text-align: right; }

.lecture-nav-label {
  font-family: var(--font-display);
  font-size: var(--text-xs);
  color: var(--color-text-faint);
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.06em;
}

.lecture-nav-title {
  font-family: var(--font-display);
  font-size: var(--text-sm);
  font-weight: 600;
  color: var(--color-text);
}

/* ============================================================
   FOOTER
   ============================================================ */
.site-footer {
  margin-top: var(--space-20);
  padding-block: var(--space-12);
  border-top: 1px solid var(--color-divider);
  text-align: center;
}

.footer-logo {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  margin-bottom: var(--space-4);
  font-family: var(--font-display);
  font-weight: 700;
  color: var(--color-text-muted);
  text-decoration: none;
}

.footer-desc {
  font-size: var(--text-sm);
  color: var(--color-text-faint);
  max-width: 40ch;
  margin-inline: auto;
  margin-bottom: var(--space-6);
}

.footer-nav {
  display: flex;
  justify-content: center;
  gap: var(--space-6);
  flex-wrap: wrap;
  list-style: none;
}

.footer-nav a {
  font-family: var(--font-display);
  font-size: var(--text-xs);
  color: var(--color-text-faint);
  text-decoration: none;
}

.footer-nav a:hover { color: var(--color-primary); }

/* ============================================================
   PROGRESS INDICATOR
   ============================================================ */
.progress-bar {
  position: fixed;
  top: 0;
  left: 0;
  height: 3px;
  background: linear-gradient(to right, var(--color-primary), var(--color-violet));
  z-index: 1000;
  transition: width 0.1s;
  pointer-events: none;
}

/* ============================================================
   OVERVIEW PAGE
   ============================================================ */
.overview-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(min(100%, 280px), 1fr));
  gap: var(--space-5);
}

.overview-card {
  display: flex;
  flex-direction: column;
  padding: var(--space-5);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  text-decoration: none;
  color: var(--color-text);
  gap: var(--space-3);
  transition: transform var(--transition), box-shadow var(--transition), border-color var(--transition);
}

.overview-card:hover {
  transform: translateY(-2px);
  box-shadow: var(--shadow-md), var(--shadow-glow);
  border-color: color-mix(in oklab, var(--color-primary) 35%, var(--color-border));
}

.overview-card-num {
  font-family: var(--font-mono);
  font-size: var(--text-xs);
  color: var(--color-primary);
  font-weight: 500;
}

.overview-card-title {
  font-family: var(--font-display);
  font-size: var(--text-base);
  font-weight: 600;
  line-height: 1.3;
}

.overview-card-topics {
  flex: 1;
  display: flex;
  flex-direction: column;
  gap: var(--space-1);
}

.overview-topic {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  font-size: var(--text-xs);
  color: var(--color-text-muted);
}

.overview-topic::before {
  content: '';
  display: inline-block;
  width: 4px;
  height: 4px;
  border-radius: 50%;
  background: var(--color-primary);
  flex-shrink: 0;
  opacity: 0.6;
}

.overview-card-footer {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding-top: var(--space-3);
  border-top: 1px solid var(--color-divider);
  font-family: var(--font-display);
  font-size: var(--text-xs);
}

.overview-count { color: var(--color-text-faint); }
.overview-link { color: var(--color-primary); font-weight: 600; }

/* ============================================================
   ACCORDION COLLAPSE
   ============================================================ */
.exercise-collapsible {
  display: none;
}

.exercise-collapsible.is-open {
  display: block;
}

/* ============================================================
   RESPONSIVE
   ============================================================ */
@media (max-width: 640px) {
  .nav-links { display: none; }
  .lecture-nav { grid-template-columns: 1fr; }
  .hero-title { font-size: clamp(1.8rem, 8vw, 2.5rem); }
  .stats-bar { gap: var(--space-8); }
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme]) {
    --color-bg:             #0d0f14;
    --color-surface:        #11141b;
    --color-surface-2:      #161a24;
    --color-surface-offset: #1c2030;
    --color-surface-3:      #202538;
    --color-divider:        #252b3f;
    --color-border:         #2d3450;
    --color-text:           #e8ecf4;
    --color-text-muted:     #8a93b2;
    --color-text-faint:     #4a5270;
    --color-text-inverse:   #0d0f14;
    --color-primary:        #00c9d7;
    --color-primary-hover:  #00a8b5;
    --color-primary-active: #008899;
    --color-primary-dim:    #003a40;
    --color-gold:           #f0c040;
    --color-gold-dim:       #3d3010;
    --color-violet:         #9b7fe8;
    --color-violet-dim:     #2a1f4a;
    --color-success:        #4dd692;
    --color-success-dim:    #0e3024;
    --color-error:          #f07070;
    --color-error-dim:      #3a1010;
    --shadow-sm: 0 1px 3px oklch(0 0 0 / 0.3);
    --shadow-md: 0 4px 16px oklch(0 0 0 / 0.4);
    --shadow-lg: 0 12px 40px oklch(0 0 0 / 0.5);
    --shadow-glow: 0 0 24px oklch(0.7 0.2 195 / 0.2);
  }
}

/* KaTeX overrides */
.katex { font-size: 1.05em !important; }
.katex-display > .katex { white-space: normal; }