/* ============================================================
   XPELION ACADEMY — MAIN STYLESHEET
   xpelion.com
   ============================================================ */

@import url('https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700;800;900&family=JetBrains+Mono:wght@400;500&display=swap');

/* ── Custom Properties ────────────────────────────────────── */
:root {
  --color-bg:        #f8faff;
  --color-bg-card:   #ffffff;
  --color-bg-alt:    #f0f4ff;
  --color-border:    #e2e8f0;
  --color-text:      #0f172a;
  --color-muted:     #64748b;
  --color-faint:     #94a3b8;

  --color-primary:   #4f46e5;
  --color-primary-h: #3730a3;
  --color-accent:    #7c3aed;

  --grad-hero:   linear-gradient(135deg, #0f172a 0%, #1e1b4b 50%, #0f172a 100%);
  --grad-math:   linear-gradient(135deg, #4f46e5, #7c3aed);
  --grad-sci:    linear-gradient(135deg, #059669, #0ea5e9);
  --grad-card:   linear-gradient(135deg, #667eea22, #764ba211);

  --shadow-sm:   0 1px 3px rgba(0,0,0,.06), 0 1px 2px rgba(0,0,0,.04);
  --shadow-md:   0 4px 16px rgba(0,0,0,.08), 0 2px 6px rgba(0,0,0,.05);
  --shadow-lg:   0 10px 40px rgba(0,0,0,.12), 0 4px 12px rgba(0,0,0,.06);
  --shadow-xl:   0 20px 60px rgba(0,0,0,.16);

  --radius-sm:   8px;
  --radius-md:   12px;
  --radius-lg:   20px;
  --radius-xl:   28px;
  --radius-full: 9999px;

  --font-sans:   'Inter', system-ui, -apple-system, sans-serif;
  --font-mono:   'JetBrains Mono', 'Fira Code', monospace;

  --nav-h:       68px;
  --max-w:       1280px;
  --transition:  .2s cubic-bezier(.4,0,.2,1);
}

/* ── Reset ────────────────────────────────────────────────── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; -webkit-text-size-adjust: 100%; }
body {
  font-family: var(--font-sans);
  background: var(--color-bg);
  color: var(--color-text);
  line-height: 1.6;
  min-height: 100vh;
}
img, svg { display: block; max-width: 100%; }
a { color: inherit; text-decoration: none; }
button { cursor: pointer; font-family: inherit; border: none; background: none; }
input, select, textarea { font-family: inherit; }
ul, ol { list-style: none; }

/* ── Typography ───────────────────────────────────────────── */
h1 { font-size: clamp(2rem, 5vw, 3.5rem); font-weight: 800; line-height: 1.1; letter-spacing: -.03em; }
h2 { font-size: clamp(1.5rem, 3vw, 2.25rem); font-weight: 700; line-height: 1.2; letter-spacing: -.02em; }
h3 { font-size: clamp(1.1rem, 2vw, 1.5rem); font-weight: 600; line-height: 1.3; }
h4 { font-size: 1.1rem; font-weight: 600; }
h5 { font-size: .95rem; font-weight: 600; }
p  { color: var(--color-muted); line-height: 1.75; }

/* ── Layout Utilities ─────────────────────────────────────── */
.container { max-width: var(--max-w); margin: 0 auto; padding: 0 24px; }
.section    { padding: 80px 0; }
.section-sm { padding: 48px 0; }
.flex   { display: flex; }
.grid   { display: grid; }
.center { text-align: center; }

/* ── NAV ──────────────────────────────────────────────────── */
#nav {
  position: fixed; top: 0; left: 0; right: 0; z-index: 1000;
  height: var(--nav-h);
  background: rgba(255,255,255,.85);
  backdrop-filter: blur(20px) saturate(180%);
  border-bottom: 1px solid var(--color-border);
  transition: box-shadow var(--transition);
}
#nav.scrolled { box-shadow: var(--shadow-md); }
.nav-inner {
  display: flex; align-items: center; gap: 32px;
  height: 100%; max-width: var(--max-w); margin: 0 auto; padding: 0 24px;
}
.nav-logo {
  display: flex; align-items: center; gap: 10px;
  font-size: 1.25rem; font-weight: 800; color: var(--color-text);
  flex-shrink: 0;
}
.nav-logo .logo-icon {
  width: 36px; height: 36px;
  background: var(--grad-math);
  border-radius: 10px;
  display: flex; align-items: center; justify-content: center;
  color: white; font-size: .9rem; font-weight: 900;
}
.nav-links {
  display: flex; align-items: center; gap: 4px; flex: 1;
}
.nav-links a {
  padding: 6px 14px;
  border-radius: var(--radius-full);
  font-size: .875rem; font-weight: 500;
  color: var(--color-muted);
  transition: all var(--transition);
}
.nav-links a:hover, .nav-links a.active {
  background: var(--color-bg-alt);
  color: var(--color-text);
}
.nav-search {
  display: flex; align-items: center; gap: 8px;
  background: var(--color-bg-alt);
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-full);
  padding: 7px 14px;
  transition: all var(--transition);
  width: 220px;
}
.nav-search:focus-within {
  border-color: var(--color-primary);
  background: white;
  box-shadow: 0 0 0 3px rgba(79,70,229,.1);
  width: 280px;
}
.nav-search svg { color: var(--color-faint); flex-shrink: 0; }
.nav-search input {
  border: none; background: none; outline: none;
  font-size: .875rem; color: var(--color-text); width: 100%;
}
.nav-search input::placeholder { color: var(--color-faint); }
.nav-cta {
  padding: 8px 20px;
  background: var(--grad-math);
  color: white;
  border-radius: var(--radius-full);
  font-size: .875rem; font-weight: 600;
  transition: transform var(--transition), box-shadow var(--transition);
  flex-shrink: 0;
}
.nav-cta:hover { transform: translateY(-1px); box-shadow: 0 6px 20px rgba(79,70,229,.4); }
.nav-menu-btn {
  display: none; width: 36px; height: 36px;
  align-items: center; justify-content: center;
  border-radius: var(--radius-sm);
  color: var(--color-muted);
}
.mobile-menu {
  display: none; position: fixed; inset: 0; z-index: 999;
  background: white; padding: 90px 24px 24px;
  flex-direction: column; gap: 8px;
  overflow-y: auto;
}
.mobile-menu.open { display: flex; }
.mobile-menu a {
  padding: 14px 18px; border-radius: var(--radius-md);
  font-weight: 500; color: var(--color-text);
  border-bottom: 1px solid var(--color-border);
}

/* ── HERO ─────────────────────────────────────────────────── */
.hero {
  background: var(--grad-hero);
  padding: calc(var(--nav-h) + 80px) 0 100px;
  text-align: center; overflow: hidden; position: relative;
}
.hero::before {
  content: '';
  position: absolute; inset: 0;
  background: radial-gradient(ellipse 80% 60% at 50% 0%, rgba(79,70,229,.3), transparent);
}
.hero::after {
  content: '';
  position: absolute; bottom: 0; left: 0; right: 0; height: 1px;
  background: linear-gradient(90deg, transparent, rgba(255,255,255,.1), transparent);
}
.hero-content { position: relative; z-index: 1; }
.hero-badge {
  display: inline-flex; align-items: center; gap: 8px;
  background: rgba(255,255,255,.08);
  border: 1px solid rgba(255,255,255,.15);
  border-radius: var(--radius-full);
  padding: 6px 16px;
  font-size: .8rem; font-weight: 600; color: rgba(255,255,255,.85);
  letter-spacing: .05em; text-transform: uppercase;
  margin-bottom: 28px;
}
.hero-badge .dot { width: 6px; height: 6px; border-radius: 50%; background: #4ade80; animation: pulse 2s infinite; }
.hero h1 { color: white; margin-bottom: 20px; }
.hero h1 span {
  background: linear-gradient(135deg, #a78bfa, #60a5fa, #34d399);
  -webkit-background-clip: text; -webkit-text-fill-color: transparent; background-clip: text;
}
.hero p { color: rgba(255,255,255,.65); font-size: 1.125rem; max-width: 600px; margin: 0 auto 40px; }
.hero-search {
  display: flex; align-items: center; gap: 0;
  background: white;
  border-radius: var(--radius-full);
  padding: 6px 6px 6px 20px;
  max-width: 540px; margin: 0 auto 48px;
  box-shadow: var(--shadow-xl);
}
.hero-search svg { color: var(--color-faint); flex-shrink: 0; margin-right: 10px; }
.hero-search input {
  flex: 1; border: none; outline: none;
  font-size: 1rem; color: var(--color-text);
}
.hero-search input::placeholder { color: var(--color-faint); }
.hero-search button {
  padding: 10px 24px;
  background: var(--grad-math);
  color: white; border-radius: var(--radius-full);
  font-weight: 600; font-size: .9rem;
  transition: transform var(--transition);
}
.hero-search button:hover { transform: scale(1.03); }
.hero-stats {
  display: flex; justify-content: center; gap: 48px;
  flex-wrap: wrap;
}
.hero-stat { color: rgba(255,255,255,.7); font-size: .875rem; }
.hero-stat strong { display: block; font-size: 1.5rem; color: white; font-weight: 800; }

/* ── SUBJECT GRID ─────────────────────────────────────────── */
.subject-grid {
  display: grid; grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: 20px; margin-top: 48px;
}
.subject-card {
  background: var(--color-bg-card);
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: 28px;
  text-decoration: none;
  transition: all var(--transition);
  position: relative; overflow: hidden;
}
.subject-card::before {
  content: '';
  position: absolute; top: 0; left: 0; right: 0; height: 3px;
  background: var(--card-accent, var(--grad-math));
}
.subject-card:hover {
  transform: translateY(-4px);
  box-shadow: var(--shadow-lg);
  border-color: transparent;
}
.card-icon {
  width: 48px; height: 48px;
  border-radius: var(--radius-md);
  display: flex; align-items: center; justify-content: center;
  font-size: 1.5rem;
  margin-bottom: 16px;
  background: var(--card-bg-icon, #f0f4ff);
}
.subject-card h3 { margin-bottom: 8px; }
.subject-card p  { font-size: .875rem; margin-bottom: 16px; }
.card-meta {
  display: flex; align-items: center; gap: 12px;
  font-size: .78rem; color: var(--color-faint); font-weight: 500;
}
.badge {
  display: inline-flex; align-items: center;
  padding: 3px 10px; border-radius: var(--radius-full);
  font-size: .72rem; font-weight: 600; letter-spacing: .03em;
}
.badge-math    { background: #eef2ff; color: #4f46e5; }
.badge-sci     { background: #ecfdf5; color: #059669; }
.badge-k12     { background: #fff7ed; color: #ea580c; }
.badge-college { background: #fdf4ff; color: #7c3aed; }
.badge-adv     { background: #fef2f2; color: #dc2626; }

/* ── TOPIC GRID ───────────────────────────────────────────── */
.topics-grid {
  display: grid; grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 16px;
}
.topic-card {
  background: var(--color-bg-card);
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 22px;
  cursor: pointer;
  transition: all var(--transition);
  text-decoration: none; display: block;
}
.topic-card:hover {
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px rgba(79,70,229,.08);
  transform: translateY(-2px);
}
.topic-card h4 { margin-bottom: 6px; }
.topic-card p  { font-size: .82rem; }
.progress-bar {
  margin-top: 14px;
  height: 4px; background: var(--color-bg-alt);
  border-radius: var(--radius-full); overflow: hidden;
}
.progress-fill {
  height: 100%; border-radius: var(--radius-full);
  background: var(--grad-math);
  transition: width .6s ease;
}

/* ── BREADCRUMB ───────────────────────────────────────────── */
.breadcrumb {
  display: flex; align-items: center; gap: 8px;
  font-size: .8rem; color: var(--color-faint);
  margin-bottom: 32px;
}
.breadcrumb a { color: var(--color-primary); }
.breadcrumb span { font-size: .6rem; }

/* ── LESSON LAYOUT ────────────────────────────────────────── */
.lesson-layout {
  display: grid; grid-template-columns: 260px 1fr;
  gap: 32px; align-items: start;
  margin-top: 32px;
}
.lesson-sidebar {
  position: sticky; top: calc(var(--nav-h) + 24px);
  background: var(--color-bg-card);
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-lg);
  overflow: hidden;
}
.sidebar-header {
  padding: 18px 20px;
  background: var(--grad-math); color: white;
  font-weight: 700; font-size: .9rem;
}
.sidebar-nav { padding: 12px; }
.sidebar-item {
  display: flex; align-items: center; gap: 10px;
  padding: 10px 12px;
  border-radius: var(--radius-sm);
  font-size: .85rem; color: var(--color-muted);
  cursor: pointer; transition: all var(--transition);
}
.sidebar-item:hover, .sidebar-item.active {
  background: var(--color-bg-alt);
  color: var(--color-text);
}
.sidebar-item .check {
  width: 18px; height: 18px;
  border: 2px solid var(--color-border);
  border-radius: 50%; flex-shrink: 0;
  display: flex; align-items: center; justify-content: center;
  font-size: .6rem;
}
.sidebar-item.done .check {
  background: #059669; border-color: #059669; color: white;
}
.lesson-content {
  background: var(--color-bg-card);
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-lg);
  overflow: hidden;
}
.lesson-header {
  padding: 36px 40px 28px;
  border-bottom: 1.5px solid var(--color-border);
}
.lesson-body { padding: 36px 40px; }
.lesson-body h3 { margin: 32px 0 14px; color: var(--color-text); }
.lesson-body h4 { margin: 24px 0 10px; }
.lesson-body p  { margin-bottom: 16px; }
.lesson-body ul, .lesson-body ol {
  padding-left: 20px; margin-bottom: 16px;
}
.lesson-body li { color: var(--color-muted); margin-bottom: 6px; line-height: 1.7; }
.lesson-body ul { list-style: disc; }
.lesson-body ol { list-style: decimal; }

/* ── CONCEPT BOX ──────────────────────────────────────────── */
.concept-box {
  background: linear-gradient(135deg, #eef2ff, #f5f3ff);
  border: 1.5px solid #c7d2fe;
  border-radius: var(--radius-md);
  padding: 24px;
  margin: 24px 0;
}
.concept-box h4 { color: var(--color-primary); margin-bottom: 10px; font-size: .9rem; text-transform: uppercase; letter-spacing: .08em; }
.concept-box p  { color: var(--color-text); margin: 0; }

/* ── FORMULA BOX ──────────────────────────────────────────── */
.formula-box {
  background: #0f172a;
  border-radius: var(--radius-md);
  padding: 20px 24px;
  margin: 20px 0;
  font-family: var(--font-mono);
  font-size: .95rem;
  color: #e2e8f0;
  overflow-x: auto;
  position: relative;
}
.formula-box .label {
  position: absolute; top: 10px; right: 12px;
  font-size: .7rem; color: #475569; font-family: var(--font-sans);
  text-transform: uppercase; letter-spacing: .08em;
}

/* ── EXAMPLE BOX ──────────────────────────────────────────── */
.example-box {
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-md);
  overflow: hidden;
  margin: 24px 0;
}
.example-box-header {
  background: var(--color-bg-alt);
  padding: 12px 20px;
  font-size: .8rem; font-weight: 700;
  color: var(--color-primary);
  text-transform: uppercase; letter-spacing: .08em;
  display: flex; align-items: center; gap: 8px;
}
.example-box-body { padding: 20px; }
.example-box-body p { margin-bottom: 10px; }
.example-solution {
  background: #f0fdf4;
  border-top: 1.5px solid #bbf7d0;
  padding: 16px 20px;
}
.example-solution strong { color: #059669; font-size: .8rem; text-transform: uppercase; letter-spacing: .05em; display: block; margin-bottom: 6px; }

/* ── QUIZ SECTION ─────────────────────────────────────────── */
.quiz-section {
  border: 2px solid var(--color-border);
  border-radius: var(--radius-lg);
  overflow: hidden;
  margin: 40px 0;
}
.quiz-header {
  background: var(--grad-math); color: white;
  padding: 20px 28px;
  display: flex; align-items: center; gap: 14px;
}
.quiz-header h3 { color: white; margin: 0; font-size: 1.1rem; }
.quiz-header p  { color: rgba(255,255,255,.75); margin: 0; font-size: .85rem; }
.quiz-icon { font-size: 1.5rem; }
.quiz-body { padding: 28px; }
.quiz-question {
  margin-bottom: 28px; padding-bottom: 28px;
  border-bottom: 1px solid var(--color-border);
}
.quiz-question:last-child { border-bottom: none; margin-bottom: 0; padding-bottom: 0; }
.question-text { font-weight: 600; color: var(--color-text); margin-bottom: 16px; font-size: .95rem; }
.quiz-options { display: flex; flex-direction: column; gap: 10px; }
.quiz-option {
  display: flex; align-items: center; gap: 12px;
  padding: 12px 16px;
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-md);
  cursor: pointer;
  transition: all var(--transition);
  font-size: .9rem;
}
.quiz-option:hover { border-color: var(--color-primary); background: #eef2ff; }
.quiz-option.selected { border-color: var(--color-primary); background: #eef2ff; }
.quiz-option.correct  { border-color: #059669; background: #f0fdf4; color: #059669; }
.quiz-option.wrong    { border-color: #dc2626; background: #fef2f2; color: #dc2626; }
.option-circle {
  width: 22px; height: 22px;
  border: 2px solid var(--color-border);
  border-radius: 50%; flex-shrink: 0;
  display: flex; align-items: center; justify-content: center;
  font-size: .7rem; font-weight: 700;
}
.quiz-option.selected .option-circle { border-color: var(--color-primary); background: var(--color-primary); color: white; }
.quiz-option.correct  .option-circle { border-color: #059669; background: #059669; color: white; }
.quiz-option.wrong    .option-circle { border-color: #dc2626; background: #dc2626; color: white; }
.quiz-explanation {
  margin-top: 12px; padding: 12px 16px;
  background: #fffbeb; border-radius: var(--radius-sm);
  font-size: .85rem; color: #92400e;
  display: none;
}
.quiz-explanation.show { display: block; }
.quiz-btn {
  margin-top: 24px;
  padding: 12px 28px;
  background: var(--grad-math);
  color: white; border-radius: var(--radius-full);
  font-weight: 600; font-size: .9rem;
  transition: transform var(--transition), box-shadow var(--transition);
}
.quiz-btn:hover { transform: translateY(-2px); box-shadow: 0 8px 24px rgba(79,70,229,.35); }
.quiz-result {
  text-align: center; padding: 24px;
  background: var(--color-bg-alt);
  border-radius: var(--radius-md);
  display: none; margin-top: 20px;
}
.quiz-result.show { display: block; }
.quiz-score { font-size: 2.5rem; font-weight: 900; color: var(--color-primary); }

/* ── HANDOUT CARDS ────────────────────────────────────────── */
.handout-card {
  background: var(--color-bg-card);
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 22px;
  display: flex; align-items: flex-start; gap: 18px;
  transition: all var(--transition);
}
.handout-card:hover { box-shadow: var(--shadow-md); border-color: transparent; }
.handout-icon {
  width: 48px; height: 48px; flex-shrink: 0;
  border-radius: var(--radius-md);
  background: var(--grad-math);
  display: flex; align-items: center; justify-content: center;
  font-size: 1.25rem;
}
.handout-info h4 { margin-bottom: 4px; }
.handout-info p  { font-size: .82rem; margin-bottom: 10px; }
.download-btn {
  display: inline-flex; align-items: center; gap: 6px;
  padding: 7px 16px;
  background: var(--color-bg-alt);
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-full);
  font-size: .78rem; font-weight: 600; color: var(--color-text);
  transition: all var(--transition); cursor: pointer;
}
.download-btn:hover { background: var(--color-primary); color: white; border-color: transparent; }

/* ── RESOURCE CARDS (AFFILIATE) ───────────────────────────── */
.resource-card {
  background: var(--color-bg-card);
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-lg);
  overflow: hidden;
  transition: all var(--transition);
}
.resource-card:hover { box-shadow: var(--shadow-lg); transform: translateY(-4px); }
.resource-img {
  height: 180px; background: var(--color-bg-alt);
  display: flex; align-items: center; justify-content: center;
  font-size: 3rem;
  border-bottom: 1px solid var(--color-border);
}
.resource-body { padding: 20px; }
.resource-body h4 { margin-bottom: 6px; font-size: .95rem; }
.resource-body p  { font-size: .82rem; margin-bottom: 14px; }
.resource-meta {
  display: flex; align-items: center; justify-content: space-between;
  margin-bottom: 14px;
}
.price { font-size: 1.1rem; font-weight: 700; color: var(--color-text); }
.stars { color: #f59e0b; font-size: .85rem; }
.amazon-btn {
  display: flex; align-items: center; justify-content: center; gap: 8px;
  width: 100%; padding: 11px;
  background: linear-gradient(135deg, #ff9900, #ff6600);
  color: white; border-radius: var(--radius-md);
  font-weight: 600; font-size: .875rem;
  transition: transform var(--transition);
}
.amazon-btn:hover { transform: scale(1.02); }

/* ── SEARCH PAGE ──────────────────────────────────────────── */
.search-result {
  background: var(--color-bg-card);
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-md);
  padding: 22px;
  margin-bottom: 16px;
  transition: all var(--transition);
  text-decoration: none; display: block;
}
.search-result:hover { border-color: var(--color-primary); box-shadow: 0 0 0 3px rgba(79,70,229,.08); }
.search-result h4 { color: var(--color-primary); margin-bottom: 6px; }
.search-result p  { font-size: .875rem; }
.search-result .path { font-size: .75rem; color: var(--color-faint); margin-top: 8px; }
mark { background: #fef9c3; padding: 0 2px; border-radius: 2px; }
.search-input-wrap {
  position: relative; margin-bottom: 32px;
}
.search-input-wrap svg {
  position: absolute; left: 18px; top: 50%;
  transform: translateY(-50%); color: var(--color-faint);
}
.search-input-wrap input {
  width: 100%; padding: 16px 18px 16px 50px;
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-full);
  font-size: 1rem; background: var(--color-bg-card);
  outline: none; transition: all var(--transition);
}
.search-input-wrap input:focus {
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px rgba(79,70,229,.1);
}

/* ── SECTION HEADER ───────────────────────────────────────── */
.section-header { margin-bottom: 48px; }
.section-header p { max-width: 560px; font-size: 1.05rem; }
.section-header.center p { margin: 0 auto; }
.section-tag {
  display: inline-flex; align-items: center; gap: 8px;
  padding: 4px 14px; border-radius: var(--radius-full);
  background: #eef2ff;
  font-size: .75rem; font-weight: 700;
  color: var(--color-primary);
  letter-spacing: .08em; text-transform: uppercase;
  margin-bottom: 14px;
}

/* ── TABS ─────────────────────────────────────────────────── */
.tabs {
  display: flex; gap: 4px;
  background: var(--color-bg-alt);
  border-radius: var(--radius-md);
  padding: 4px; margin-bottom: 32px;
  width: fit-content;
}
.tab-btn {
  padding: 9px 20px;
  border-radius: var(--radius-sm);
  font-size: .875rem; font-weight: 500;
  color: var(--color-muted);
  transition: all var(--transition);
}
.tab-btn.active {
  background: white; color: var(--color-text);
  box-shadow: var(--shadow-sm);
}
.tab-panel { display: none; }
.tab-panel.active { display: block; }

/* ── FILTER CHIPS ─────────────────────────────────────────── */
.filter-chips {
  display: flex; gap: 8px; flex-wrap: wrap; margin-bottom: 28px;
}
.chip {
  padding: 6px 16px;
  border: 1.5px solid var(--color-border);
  border-radius: var(--radius-full);
  font-size: .8rem; font-weight: 500;
  color: var(--color-muted);
  cursor: pointer; transition: all var(--transition);
}
.chip:hover, .chip.active {
  border-color: var(--color-primary);
  background: #eef2ff; color: var(--color-primary);
}

/* ── FOOTER ───────────────────────────────────────────────── */
footer {
  background: #0f172a; color: rgba(255,255,255,.7);
  padding: 64px 0 32px;
}
.footer-grid {
  display: grid; grid-template-columns: 2fr 1fr 1fr 1fr;
  gap: 48px; margin-bottom: 48px;
}
.footer-brand .nav-logo { margin-bottom: 16px; color: white; }
.footer-brand p { font-size: .875rem; max-width: 280px; }
.footer-col h5 { color: white; margin-bottom: 16px; font-size: .85rem; text-transform: uppercase; letter-spacing: .08em; }
.footer-col a {
  display: block; font-size: .875rem;
  margin-bottom: 10px; transition: color var(--transition);
}
.footer-col a:hover { color: white; }
.footer-bottom {
  border-top: 1px solid rgba(255,255,255,.1);
  padding-top: 24px;
  display: flex; justify-content: space-between; align-items: center;
  font-size: .8rem;
}
.footer-bottom a { margin-left: 20px; }
.footer-bottom a:hover { color: white; }

/* ── ANIMATIONS ───────────────────────────────────────────── */
@keyframes pulse { 0%,100%{opacity:1} 50%{opacity:.4} }
@keyframes fadeUp {
  from { opacity:0; transform:translateY(24px); }
  to   { opacity:1; transform:translateY(0); }
}
.fade-up { animation: fadeUp .5s ease forwards; }
.fade-up-d1 { animation-delay: .1s; opacity: 0; }
.fade-up-d2 { animation-delay: .2s; opacity: 0; }
.fade-up-d3 { animation-delay: .3s; opacity: 0; }

/* ── UTILITY ──────────────────────────────────────────────── */
.text-gradient {
  background: var(--grad-math);
  -webkit-background-clip: text; -webkit-text-fill-color: transparent; background-clip: text;
}
.text-grad-sci {
  background: var(--grad-sci);
  -webkit-background-clip: text; -webkit-text-fill-color: transparent; background-clip: text;
}
.btn {
  display: inline-flex; align-items: center; gap: 8px;
  padding: 11px 24px; border-radius: var(--radius-full);
  font-weight: 600; font-size: .9rem;
  transition: all var(--transition);
}
.btn-primary {
  background: var(--grad-math); color: white;
}
.btn-primary:hover { transform: translateY(-2px); box-shadow: 0 8px 24px rgba(79,70,229,.35); }
.btn-outline {
  border: 1.5px solid var(--color-border); color: var(--color-text);
}
.btn-outline:hover { border-color: var(--color-primary); color: var(--color-primary); background: #eef2ff; }
.mt-8  { margin-top: 8px; }
.mt-16 { margin-top: 16px; }
.mt-24 { margin-top: 24px; }
.mt-32 { margin-top: 32px; }
.mb-8  { margin-bottom: 8px; }
.mb-16 { margin-bottom: 16px; }
.mb-24 { margin-bottom: 24px; }
.divider { height: 1px; background: var(--color-border); margin: 32px 0; }

/* ── INFO BANNER ──────────────────────────────────────────── */
.info-banner {
  background: #fffbeb; border: 1.5px solid #fde68a;
  border-radius: var(--radius-md); padding: 14px 18px;
  font-size: .85rem; color: #92400e;
  margin-bottom: 24px;
  display: flex; align-items: flex-start; gap: 10px;
}

/* ── RESPONSIVE ───────────────────────────────────────────── */
@media (max-width: 1024px) {
  .footer-grid { grid-template-columns: 1fr 1fr; }
  .lesson-layout { grid-template-columns: 1fr; }
  .lesson-sidebar { position: static; }
}
@media (max-width: 768px) {
  .nav-links, .nav-search { display: none; }
  .nav-menu-btn { display: flex; }
  .hero { padding: calc(var(--nav-h) + 48px) 0 64px; }
  .subject-grid { grid-template-columns: 1fr; }
  .topics-grid  { grid-template-columns: 1fr; }
  .footer-grid  { grid-template-columns: 1fr; gap: 32px; }
  .footer-bottom { flex-direction: column; gap: 12px; text-align: center; }
  .lesson-header, .lesson-body { padding: 24px; }
  .hero-stats { gap: 24px; }
  .tabs { width: 100%; overflow-x: auto; }
}
@media (max-width: 480px) {
  .container { padding: 0 16px; }
  .section { padding: 56px 0; }
}