/* ═══════════════════════════════════════════
   CSS VARIABLES & THEMES
═══════════════════════════════════════════ */
:root {
  --font-display: 'Syne', sans-serif;
  --font-body: 'DM Sans', sans-serif;
  --font-code: 'JetBrains Mono', monospace;

  --react: #61dafb;
  --vue: #42d392;
  --accent: #f7c948;
  --danger: #ff6b6b;

  --sidebar-w: 280px;
  --topbar-h: 56px;

  --radius: 10px;
  --radius-lg: 16px;
  --shadow: 0 4px 24px rgba(0,0,0,0.25);
  --shadow-sm: 0 2px 8px rgba(0,0,0,0.15);

  --transition: 0.2s ease;
  --transition-slow: 0.4s cubic-bezier(0.16,1,0.3,1);
}

[data-theme="dark"] {
  --bg: #0d0f12;
  --bg-2: #14171c;
  --bg-3: #1c2028;
  --bg-4: #242831;
  --surface: #1c2028;
  --surface-2: #242831;
  --surface-3: #2c3140;
  --border: rgba(255,255,255,0.07);
  --border-2: rgba(255,255,255,0.12);
  --text: #e8ecf0;
  --text-2: #9aa3b0;
  --text-3: #5a6475;
  --code-bg: #0a0c0f;
  --react-bg: rgba(97,218,251,0.08);
  --vue-bg: rgba(66,211,146,0.08);
  --react-border: rgba(97,218,251,0.2);
  --vue-border: rgba(66,211,146,0.2);
}

[data-theme="light"] {
  --bg: #f5f7fa;
  --bg-2: #ffffff;
  --bg-3: #eef1f6;
  --bg-4: #e4e8f0;
  --surface: #ffffff;
  --surface-2: #f0f3f8;
  --surface-3: #e4e8f0;
  --border: rgba(0,0,0,0.08);
  --border-2: rgba(0,0,0,0.14);
  --text: #1a1d23;
  --text-2: #4a5568;
  --text-3: #9aa3b0;
  --code-bg: #1c2028;
  --react-bg: rgba(97,218,251,0.07);
  --vue-bg: rgba(66,211,146,0.07);
  --react-border: rgba(97,218,251,0.3);
  --vue-border: rgba(66,211,146,0.3);
}

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

html { scroll-behavior: smooth; }

body {
  font-family: var(--font-body);
  background: var(--bg);
  color: var(--text);
  display: flex;
  min-height: 100vh;
  overflow-x: hidden;
  transition: background var(--transition), color var(--transition);
}

/* ═══════════════════════════════════════════
   SIDEBAR
═══════════════════════════════════════════ */
.sidebar {
  width: var(--sidebar-w);
  min-height: 100vh;
  background: var(--bg-2);
  border-right: 1px solid var(--border);
  display: flex;
  flex-direction: column;
  position: fixed;
  top: 0; left: 0; bottom: 0;
  z-index: 100;
  overflow-y: auto;
  transition: transform var(--transition-slow), background var(--transition);
  scrollbar-width: thin;
  scrollbar-color: var(--border-2) transparent;
}

.sidebar-header {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 20px 18px;
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  background: var(--bg-2);
  z-index: 10;
}

.logo {
  display: flex;
  align-items: center;
  gap: 4px;
  font-size: 1.1rem;
}
.logo-react { color: var(--react); }
.logo-arrow { color: var(--text-3); font-size: 0.85rem; }
.logo-vue { color: var(--vue); }

.logo-text {
  font-family: var(--font-display);
  font-weight: 700;
  font-size: 0.85rem;
  color: var(--text);
  flex: 1;
}

.sidebar-toggle {
  display: none;
  background: none;
  border: none;
  color: var(--text-2);
  font-size: 1.2rem;
  cursor: pointer;
  padding: 4px;
  border-radius: 6px;
  transition: background var(--transition);
}
.sidebar-toggle:hover { background: var(--surface-3); }

/* Progress Overview */
.progress-overview {
  padding: 16px 18px;
  border-bottom: 1px solid var(--border);
}
.progress-label {
  display: flex;
  justify-content: space-between;
  font-size: 0.72rem;
  color: var(--text-2);
  margin-bottom: 8px;
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}
#progressPercent { color: var(--vue); font-weight: 700; }
.progress-bar-outer {
  height: 5px;
  background: var(--surface-3);
  border-radius: 99px;
  overflow: hidden;
}
.progress-bar-inner {
  height: 100%;
  background: linear-gradient(90deg, var(--vue), var(--react));
  border-radius: 99px;
  width: 0%;
  transition: width 0.5s cubic-bezier(0.16,1,0.3,1);
}
.progress-stats {
  font-size: 0.7rem;
  color: var(--text-3);
  margin-top: 6px;
  text-align: right;
}

/* Sidebar Nav */
.sidebar-nav { flex: 1; padding: 8px 0 80px; }

.nav-section-title {
  font-family: var(--font-display);
  font-size: 0.65rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--text-3);
  padding: 14px 18px 6px;
}

.nav-item {
  display: flex;
  align-items: center;
  gap: 9px;
  padding: 8px 18px;
  font-size: 0.825rem;
  color: var(--text-2);
  cursor: pointer;
  border-radius: 0;
  transition: all var(--transition);
  border-left: 2px solid transparent;
  position: relative;
}
.nav-item:hover {
  background: var(--surface-2);
  color: var(--text);
}
.nav-item.active {
  background: var(--surface-2);
  color: var(--text);
  border-left-color: var(--vue);
}
.nav-item.active .nav-icon { color: var(--vue); }

.nav-icon { font-size: 0.9rem; flex-shrink: 0; }

.nav-check {
  margin-left: auto;
  width: 16px;
  height: 16px;
  border-radius: 50%;
  border: 1.5px solid var(--border-2);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 0.6rem;
  flex-shrink: 0;
  transition: all var(--transition);
}
.nav-item.completed .nav-check {
  background: var(--vue);
  border-color: var(--vue);
  color: #fff;
}
.nav-item.completed .nav-check::after { content: '✓'; }

.nav-challenge-item {
  display: flex;
  align-items: center;
  gap: 9px;
  padding: 10px 18px;
  font-size: 0.825rem;
  color: var(--accent);
  cursor: pointer;
  border-radius: 0;
  transition: all var(--transition);
  border-left: 2px solid transparent;
  font-weight: 600;
}
.nav-challenge-item:hover { background: rgba(247,201,72,0.07); border-left-color: var(--accent); }

/* Sidebar Footer */
.sidebar-footer {
  position: fixed;
  bottom: 0; left: 0;
  width: var(--sidebar-w);
  padding: 12px 18px;
  background: var(--bg-2);
  border-top: 1px solid var(--border);
  z-index: 10;
}
.theme-toggle {
  display: flex;
  align-items: center;
  gap: 8px;
  background: var(--surface-2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 8px 14px;
  color: var(--text-2);
  font-size: 0.8rem;
  cursor: pointer;
  width: 100%;
  transition: all var(--transition);
  font-family: var(--font-body);
}
.theme-toggle:hover { background: var(--surface-3); color: var(--text); }

/* ═══════════════════════════════════════════
   MAIN CONTENT
═══════════════════════════════════════════ */
.main-content {
  margin-left: var(--sidebar-w);
  flex: 1;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

/* ═══════════════════════════════════════════
   HERO SCREEN
═══════════════════════════════════════════ */
.hero-screen {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  position: relative;
  overflow: hidden;
  background: var(--bg);
}
.hero-bg-grid {
  position: absolute;
  inset: 0;
  background-image:
    linear-gradient(var(--border) 1px, transparent 1px),
    linear-gradient(90deg, var(--border) 1px, transparent 1px);
  background-size: 48px 48px;
  mask-image: radial-gradient(ellipse 80% 80% at 50% 50%, black, transparent);
  pointer-events: none;
}
.hero-inner {
  position: relative;
  text-align: center;
  max-width: 640px;
  padding: 40px 24px;
  animation: fadeUp 0.7s cubic-bezier(0.16,1,0.3,1) both;
}

.hero-badge {
  display: inline-block;
  background: var(--surface-2);
  border: 1px solid var(--border-2);
  border-radius: 99px;
  padding: 5px 14px;
  font-size: 0.75rem;
  color: var(--text-2);
  letter-spacing: 0.05em;
  text-transform: uppercase;
  font-weight: 600;
  margin-bottom: 24px;
}

.hero-title {
  font-family: var(--font-display);
  font-size: clamp(3rem, 8vw, 5.5rem);
  font-weight: 800;
  line-height: 1;
  margin-bottom: 20px;
  letter-spacing: -0.03em;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 16px;
  flex-wrap: wrap;
}
.hero-react { color: var(--react); }
.hero-arrow { color: var(--text-3); font-size: 0.7em; }
.hero-vue { color: var(--vue); }

.hero-subtitle {
  font-size: 1.05rem;
  color: var(--text-2);
  line-height: 1.7;
  margin-bottom: 32px;
}

.hero-stats {
  display: flex;
  justify-content: center;
  gap: 32px;
  margin-bottom: 36px;
  flex-wrap: wrap;
}
.hero-stat {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 2px;
}
.hero-stat strong {
  font-family: var(--font-display);
  font-size: 2rem;
  font-weight: 800;
  background: linear-gradient(135deg, var(--react), var(--vue));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}
.hero-stat span { font-size: 0.75rem; color: var(--text-3); text-transform: uppercase; letter-spacing: 0.06em; }

.hero-cta {
  display: inline-block;
  background: linear-gradient(135deg, var(--vue) 0%, var(--react) 100%);
  color: #0a0c0f;
  font-family: var(--font-display);
  font-weight: 700;
  font-size: 1rem;
  padding: 14px 36px;
  border-radius: var(--radius);
  border: none;
  cursor: pointer;
  transition: transform 0.2s, box-shadow 0.2s;
  letter-spacing: 0.01em;
}
.hero-cta:hover {
  transform: translateY(-2px);
  box-shadow: 0 8px 32px rgba(66,211,146,0.3);
}
.hero-scroll-hint {
  margin-top: 16px;
  font-size: 0.8rem;
  color: var(--text-3);
}

/* ═══════════════════════════════════════════
   LESSON TOPBAR
═══════════════════════════════════════════ */
.lesson-area { padding: 0 0 80px; }

.lesson-topbar {
  display: flex;
  align-items: center;
  gap: 16px;
  padding: 14px 32px;
  border-bottom: 1px solid var(--border);
  background: var(--bg-2);
  position: sticky;
  top: 0;
  z-index: 50;
  backdrop-filter: blur(8px);
}

.back-btn {
  background: none;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  color: var(--text-2);
  padding: 7px 14px;
  font-size: 0.8rem;
  cursor: pointer;
  transition: all var(--transition);
  font-family: var(--font-body);
  white-space: nowrap;
}
.back-btn:hover { background: var(--surface-2); color: var(--text); }

.lesson-breadcrumb {
  font-size: 0.8rem;
  color: var(--text-3);
  flex: 1;
}
.lesson-breadcrumb span { color: var(--text-2); }

.mark-complete-btn {
  background: var(--surface-2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  color: var(--text-2);
  padding: 7px 16px;
  font-size: 0.8rem;
  cursor: pointer;
  transition: all var(--transition);
  font-family: var(--font-body);
  white-space: nowrap;
}
.mark-complete-btn:hover { background: var(--vue); color: #0a0c0f; border-color: var(--vue); }
.mark-complete-btn.completed { background: var(--vue); color: #0a0c0f; border-color: var(--vue); }

/* ═══════════════════════════════════════════
   LESSON BODY
═══════════════════════════════════════════ */
.lesson-body {
  max-width: 960px;
  margin: 0 auto;
  padding: 40px 32px;
  animation: fadeUp 0.4s cubic-bezier(0.16,1,0.3,1) both;
}

.lesson-header { margin-bottom: 32px; }

.lesson-category {
  font-size: 0.72rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--vue);
  margin-bottom: 8px;
}

.lesson-title {
  font-family: var(--font-display);
  font-size: clamp(1.8rem, 4vw, 2.8rem);
  font-weight: 800;
  line-height: 1.15;
  letter-spacing: -0.02em;
  margin-bottom: 14px;
}

.lesson-meta {
  display: flex;
  align-items: center;
  gap: 16px;
  flex-wrap: wrap;
}

.lesson-badge {
  display: inline-flex;
  align-items: center;
  gap: 5px;
  padding: 4px 10px;
  border-radius: 99px;
  font-size: 0.7rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.05em;
}
.badge-beginner { background: rgba(66,211,146,0.12); color: var(--vue); }
.badge-intermediate { background: rgba(97,218,251,0.12); color: var(--react); }
.badge-advanced { background: rgba(247,201,72,0.12); color: var(--accent); }

.lesson-duration { font-size: 0.78rem; color: var(--text-3); }

/* Content blocks */
.content-intro {
  font-size: 1.05rem;
  color: var(--text-2);
  line-height: 1.75;
  margin-bottom: 32px;
  padding-bottom: 32px;
  border-bottom: 1px solid var(--border);
}
.content-intro strong { color: var(--text); }

/* Concept cards */
.concept-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 16px;
  margin: 24px 0;
}
.concept-card {
  border-radius: var(--radius-lg);
  padding: 20px;
  border: 1px solid;
  transition: transform var(--transition), box-shadow var(--transition);
}
.concept-card:hover { transform: translateY(-2px); box-shadow: var(--shadow); }
.concept-card.react { background: var(--react-bg); border-color: var(--react-border); }
.concept-card.vue { background: var(--vue-bg); border-color: var(--vue-border); }
.concept-card-label {
  font-size: 0.65rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  margin-bottom: 8px;
}
.concept-card.react .concept-card-label { color: var(--react); }
.concept-card.vue .concept-card-label { color: var(--vue); }
.concept-card-title {
  font-family: var(--font-display);
  font-size: 1rem;
  font-weight: 700;
  margin-bottom: 8px;
}
.concept-card p { font-size: 0.85rem; color: var(--text-2); line-height: 1.6; }

/* Section headings */
.section-heading {
  font-family: var(--font-display);
  font-size: 1.3rem;
  font-weight: 700;
  margin: 36px 0 16px;
  display: flex;
  align-items: center;
  gap: 10px;
}
.section-heading::after {
  content: '';
  flex: 1;
  height: 1px;
  background: var(--border);
}

/* Code comparison block */
.code-comparison {
  margin: 24px 0;
  border-radius: var(--radius-lg);
  overflow: hidden;
  border: 1px solid var(--border);
  background: var(--code-bg);
}
.code-comparison-header {
  display: grid;
  grid-template-columns: 1fr 1fr;
}
.code-tab {
  padding: 10px 18px;
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  border-bottom: 2px solid transparent;
  display: flex;
  align-items: center;
  gap: 7px;
}
.code-tab.react { background: var(--react-bg); border-bottom-color: var(--react); color: var(--react); }
.code-tab.vue { background: var(--vue-bg); border-bottom-color: var(--vue); color: var(--vue); }

.code-panels {
  display: grid;
  grid-template-columns: 1fr 1fr;
  border-top: 1px solid var(--border);
}
.code-panel {
  position: relative;
  border-left: 1px solid var(--border);
}
.code-panel:first-child { border-left: none; }

.code-copy-btn {
  position: absolute;
  top: 8px; right: 8px;
  background: var(--surface-3);
  border: 1px solid var(--border);
  border-radius: 6px;
  color: var(--text-3);
  font-size: 0.7rem;
  padding: 4px 9px;
  cursor: pointer;
  opacity: 0;
  transition: all var(--transition);
  font-family: var(--font-code);
}
.code-panel:hover .code-copy-btn { opacity: 1; }
.code-copy-btn:hover { background: var(--vue); color: #0a0c0f; border-color: var(--vue); }
.code-copy-btn.copied { background: var(--vue); color: #0a0c0f; border-color: var(--vue); }

pre {
  font-family: var(--font-code);
  font-size: 0.78rem;
  line-height: 1.7;
  padding: 20px;
  overflow-x: auto;
  color: #c9d1d9;
  tab-size: 2;
  white-space: pre;
}

/* Syntax highlighting */
.kw { color: #ff7b72; }         /* keywords */
.fn { color: #d2a8ff; }         /* functions */
.str { color: #a5d6ff; }        /* strings */
.num { color: #f2cc60; }        /* numbers */
.cmt { color: #8b949e; font-style: italic; } /* comments */
.tag { color: #7ee787; }        /* HTML tags */
.attr { color: #79c0ff; }       /* attributes */
.vue-kw { color: #42d392; }     /* Vue keywords */
.react-kw { color: #61dafb; }   /* React keywords */
.op { color: #ff7b72; }         /* operators */
.type { color: #f2cc60; }       /* types */
.bool { color: #79c0ff; }       /* booleans */

/* Explanation / callout blocks */
.callout {
  border-radius: var(--radius);
  padding: 16px 20px;
  margin: 20px 0;
  display: flex;
  gap: 14px;
  align-items: flex-start;
  border: 1px solid;
}
.callout-icon { font-size: 1.1rem; flex-shrink: 0; margin-top: 1px; }
.callout-body { flex: 1; }
.callout-title { font-weight: 700; font-size: 0.85rem; margin-bottom: 5px; }
.callout p { font-size: 0.85rem; line-height: 1.65; color: var(--text-2); }
.callout.tip { background: rgba(66,211,146,0.06); border-color: rgba(66,211,146,0.25); }
.callout.tip .callout-title { color: var(--vue); }
.callout.warning { background: rgba(247,201,72,0.06); border-color: rgba(247,201,72,0.25); }
.callout.warning .callout-title { color: var(--accent); }
.callout.danger { background: rgba(255,107,107,0.06); border-color: rgba(255,107,107,0.25); }
.callout.danger .callout-title { color: var(--danger); }
.callout.info { background: rgba(97,218,251,0.06); border-color: rgba(97,218,251,0.25); }
.callout.info .callout-title { color: var(--react); }

/* Expand/collapse explanation */
.expandable {
  border: 1px solid var(--border);
  border-radius: var(--radius);
  margin: 16px 0;
  overflow: hidden;
  transition: all var(--transition);
}
.expandable-trigger {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 14px 18px;
  cursor: pointer;
  font-weight: 600;
  font-size: 0.875rem;
  background: var(--surface-2);
  transition: background var(--transition);
  user-select: none;
}
.expandable-trigger:hover { background: var(--surface-3); }
.expandable-arrow {
  transition: transform var(--transition);
  color: var(--text-3);
  font-size: 0.7rem;
}
.expandable.open .expandable-arrow { transform: rotate(180deg); }
.expandable-body {
  display: none;
  padding: 16px 18px;
  font-size: 0.875rem;
  color: var(--text-2);
  line-height: 1.7;
  border-top: 1px solid var(--border);
}
.expandable-body ul { padding-left: 20px; }
.expandable-body li { margin-bottom: 6px; }
.expandable.open .expandable-body { display: block; }

/* Key differences table */
.diff-table {
  width: 100%;
  border-collapse: collapse;
  margin: 24px 0;
  font-size: 0.85rem;
  border-radius: var(--radius);
  overflow: hidden;
  border: 1px solid var(--border);
}
.diff-table th {
  background: var(--surface-2);
  padding: 10px 16px;
  text-align: left;
  font-weight: 700;
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-2);
}
.diff-table td {
  padding: 10px 16px;
  border-top: 1px solid var(--border);
  color: var(--text-2);
  vertical-align: top;
  line-height: 1.5;
}
.diff-table tr:hover td { background: var(--surface-2); }
.diff-table .react-col { color: var(--react); font-family: var(--font-code); font-size: 0.78rem; }
.diff-table .vue-col { color: var(--vue); font-family: var(--font-code); font-size: 0.78rem; }

/* ═══════════════════════════════════════════
   QUIZ
═══════════════════════════════════════════ */
.quiz-block {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  padding: 24px;
  margin: 32px 0;
}
.quiz-header {
  display: flex;
  align-items: center;
  gap: 10px;
  margin-bottom: 16px;
}
.quiz-icon {
  width: 32px; height: 32px;
  background: rgba(247,201,72,0.12);
  border-radius: 8px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1rem;
  flex-shrink: 0;
}
.quiz-title {
  font-family: var(--font-display);
  font-weight: 700;
  font-size: 0.9rem;
  color: var(--accent);
}
.quiz-question {
  font-size: 0.95rem;
  font-weight: 600;
  color: var(--text);
  margin-bottom: 16px;
  line-height: 1.5;
}
.quiz-options { display: flex; flex-direction: column; gap: 8px; }
.quiz-option {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 12px 16px;
  background: var(--surface-2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  cursor: pointer;
  font-size: 0.875rem;
  transition: all var(--transition);
  user-select: none;
}
.quiz-option:hover { border-color: var(--border-2); background: var(--surface-3); }
.quiz-option.correct { background: rgba(66,211,146,0.1); border-color: var(--vue); color: var(--vue); }
.quiz-option.incorrect { background: rgba(255,107,107,0.1); border-color: var(--danger); color: var(--danger); }
.quiz-option.disabled { cursor: default; }
.quiz-option-letter {
  width: 24px; height: 24px;
  border-radius: 50%;
  background: var(--surface-3);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 0.7rem;
  font-weight: 700;
  flex-shrink: 0;
  font-family: var(--font-code);
}
.quiz-feedback {
  margin-top: 14px;
  padding: 12px 16px;
  border-radius: var(--radius);
  font-size: 0.85rem;
  line-height: 1.6;
  display: none;
}
.quiz-feedback.show { display: block; }
.quiz-feedback.correct { background: rgba(66,211,146,0.1); color: var(--vue); }
.quiz-feedback.incorrect { background: rgba(255,107,107,0.1); color: var(--danger); }

/* ═══════════════════════════════════════════
   TRY IT YOURSELF
═══════════════════════════════════════════ */
.tryit-block {
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  overflow: hidden;
  margin: 32px 0;
}
.tryit-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 12px 18px;
  background: var(--surface-2);
  border-bottom: 1px solid var(--border);
}
.tryit-title {
  display: flex;
  align-items: center;
  gap: 8px;
  font-weight: 700;
  font-size: 0.85rem;
  color: var(--accent);
}
.tryit-actions { display: flex; gap: 8px; }
.tryit-btn {
  background: var(--surface-3);
  border: 1px solid var(--border);
  border-radius: 6px;
  color: var(--text-2);
  font-size: 0.75rem;
  padding: 5px 12px;
  cursor: pointer;
  transition: all var(--transition);
  font-family: var(--font-body);
}
.tryit-btn:hover { background: var(--vue); color: #0a0c0f; border-color: var(--vue); }
.tryit-btn.run { background: var(--vue); color: #0a0c0f; border-color: var(--vue); }
.tryit-btn.run:hover { opacity: 0.85; }
.tryit-editor {
  font-family: var(--font-code);
  font-size: 0.78rem;
  line-height: 1.7;
  padding: 20px;
  background: var(--code-bg);
  color: #c9d1d9;
  border: none;
  width: 100%;
  min-height: 180px;
  resize: vertical;
  outline: none;
  tab-size: 2;
}
.tryit-output {
  border-top: 1px solid var(--border);
  padding: 14px 18px;
  font-family: var(--font-code);
  font-size: 0.78rem;
  color: var(--text-2);
  background: var(--bg-3);
  min-height: 48px;
  display: none;
}
.tryit-output.show { display: block; }

/* ═══════════════════════════════════════════
   PITFALLS
═══════════════════════════════════════════ */
.pitfall-list { display: flex; flex-direction: column; gap: 12px; margin: 20px 0; }
.pitfall-item {
  border-radius: var(--radius);
  padding: 16px 18px;
  background: rgba(255,107,107,0.05);
  border: 1px solid rgba(255,107,107,0.18);
  display: flex;
  gap: 12px;
  align-items: flex-start;
}
.pitfall-icon { font-size: 1.1rem; flex-shrink: 0; }
.pitfall-title { font-weight: 700; font-size: 0.875rem; color: var(--danger); margin-bottom: 4px; }
.pitfall-desc { font-size: 0.825rem; color: var(--text-2); line-height: 1.6; }

/* ═══════════════════════════════════════════
   LESSON FOOTER NAV
═══════════════════════════════════════════ */
.lesson-nav-footer {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 24px 32px;
  border-top: 1px solid var(--border);
  max-width: 960px;
  margin: 0 auto;
  width: 100%;
}
.lesson-nav-btn {
  background: var(--surface-2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  color: var(--text-2);
  padding: 10px 22px;
  font-size: 0.875rem;
  cursor: pointer;
  transition: all var(--transition);
  font-family: var(--font-body);
}
.lesson-nav-btn:hover { background: var(--surface-3); color: var(--text); }
.lesson-nav-btn.primary { background: var(--vue); color: #0a0c0f; border-color: var(--vue); font-weight: 600; }
.lesson-nav-btn.primary:hover { opacity: 0.88; }
.lesson-nav-btn:disabled { opacity: 0.3; cursor: default; }

/* ═══════════════════════════════════════════
   CHALLENGE SCREEN
═══════════════════════════════════════════ */
.challenge-screen {
  min-height: 100vh;
  padding: 40px 32px;
  animation: fadeUp 0.4s cubic-bezier(0.16,1,0.3,1) both;
}
.challenge-inner { max-width: 960px; margin: 0 auto; }

.challenge-header {
  text-align: center;
  margin-bottom: 48px;
}
.challenge-badge {
  display: inline-block;
  background: rgba(247,201,72,0.12);
  border: 1px solid rgba(247,201,72,0.3);
  border-radius: 99px;
  padding: 5px 14px;
  font-size: 0.72rem;
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: 0.08em;
  font-weight: 700;
  margin-bottom: 16px;
}
.challenge-title {
  font-family: var(--font-display);
  font-size: 2.5rem;
  font-weight: 800;
  margin-bottom: 12px;
  letter-spacing: -0.02em;
}
.challenge-subtitle { font-size: 1rem; color: var(--text-2); max-width: 500px; margin: 0 auto; line-height: 1.6; }

.challenge-steps { display: flex; flex-direction: column; gap: 24px; }

.challenge-step {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius-lg);
  overflow: hidden;
  transition: border-color var(--transition);
}
.challenge-step.active { border-color: var(--vue); }
.challenge-step.done { border-color: rgba(66,211,146,0.3); }

.challenge-step-header {
  display: flex;
  align-items: center;
  gap: 14px;
  padding: 18px 22px;
  background: var(--surface-2);
  cursor: pointer;
  user-select: none;
}
.step-num {
  width: 32px; height: 32px;
  border-radius: 50%;
  background: var(--surface-3);
  border: 2px solid var(--border);
  display: flex;
  align-items: center;
  justify-content: center;
  font-weight: 700;
  font-size: 0.85rem;
  flex-shrink: 0;
  font-family: var(--font-display);
  transition: all var(--transition);
}
.challenge-step.active .step-num { background: var(--vue); border-color: var(--vue); color: #0a0c0f; }
.challenge-step.done .step-num { background: var(--vue); border-color: var(--vue); color: #0a0c0f; }
.challenge-step.done .step-num::after { content: '✓'; }
.step-title { font-weight: 700; font-size: 0.9rem; }
.step-status { margin-left: auto; font-size: 0.75rem; color: var(--text-3); }

.challenge-step-body { display: none; padding: 22px; border-top: 1px solid var(--border); }
.challenge-step.active .challenge-step-body,
.challenge-step.open .challenge-step-body { display: block; }

.step-task { font-size: 0.9rem; color: var(--text-2); line-height: 1.7; margin-bottom: 18px; }
.step-task strong { color: var(--text); }

.reveal-btn {
  background: var(--surface-2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  color: var(--accent);
  padding: 8px 18px;
  font-size: 0.8rem;
  cursor: pointer;
  transition: all var(--transition);
  font-family: var(--font-body);
  font-weight: 600;
  margin-top: 12px;
}
.reveal-btn:hover { background: rgba(247,201,72,0.1); border-color: var(--accent); }

.step-complete-btn {
  margin-top: 16px;
  background: var(--vue);
  border: none;
  border-radius: var(--radius);
  color: #0a0c0f;
  padding: 10px 22px;
  font-size: 0.875rem;
  cursor: pointer;
  font-family: var(--font-body);
  font-weight: 700;
  transition: opacity var(--transition);
}
.step-complete-btn:hover { opacity: 0.85; }

/* ═══════════════════════════════════════════
   CERTIFICATE SCREEN
═══════════════════════════════════════════ */
.cert-screen {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 40px;
  background: var(--bg);
}
.cert-inner {
  max-width: 600px;
  width: 100%;
  text-align: center;
  animation: certReveal 0.8s cubic-bezier(0.16,1,0.3,1) both;
}

@keyframes certReveal {
  from { opacity: 0; transform: scale(0.9) translateY(30px); }
  to { opacity: 1; transform: scale(1) translateY(0); }
}

.cert-card {
  background: var(--surface);
  border: 2px solid;
  border-image: linear-gradient(135deg, var(--vue), var(--react)) 1;
  border-radius: var(--radius-lg);
  padding: 48px 40px;
  margin-bottom: 24px;
  position: relative;
  overflow: hidden;
}
.cert-card::before {
  content: '';
  position: absolute;
  inset: 0;
  background: radial-gradient(ellipse 60% 50% at 50% 0%, rgba(66,211,146,0.07), transparent);
  pointer-events: none;
}
.cert-emoji { font-size: 3.5rem; margin-bottom: 16px; }
.cert-heading {
  font-family: var(--font-display);
  font-size: 1.8rem;
  font-weight: 800;
  margin-bottom: 10px;
  background: linear-gradient(135deg, var(--vue), var(--react));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}
.cert-sub { font-size: 1rem; color: var(--text-2); margin-bottom: 24px; line-height: 1.6; }
.cert-stats {
  display: flex;
  justify-content: center;
  gap: 24px;
  margin-bottom: 24px;
  flex-wrap: wrap;
}
.cert-stat {
  background: var(--surface-2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 12px 20px;
  text-align: center;
}
.cert-stat strong {
  display: block;
  font-family: var(--font-display);
  font-size: 1.5rem;
  font-weight: 800;
  background: linear-gradient(135deg, var(--vue), var(--react));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}
.cert-stat span { font-size: 0.72rem; color: var(--text-3); text-transform: uppercase; letter-spacing: 0.06em; }
.cert-cta {
  display: inline-block;
  background: linear-gradient(135deg, var(--vue), var(--react));
  color: #0a0c0f;
  font-family: var(--font-display);
  font-weight: 700;
  font-size: 0.95rem;
  padding: 12px 28px;
  border-radius: var(--radius);
  border: none;
  cursor: pointer;
  transition: opacity var(--transition), transform var(--transition);
}
.cert-cta:hover { opacity: 0.9; transform: translateY(-1px); }

/* ═══════════════════════════════════════════
   TOAST
═══════════════════════════════════════════ */
.toast-container {
  position: fixed;
  bottom: 24px; right: 24px;
  z-index: 9999;
  display: flex;
  flex-direction: column;
  gap: 8px;
}
.toast {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 12px 18px;
  font-size: 0.85rem;
  color: var(--text);
  box-shadow: var(--shadow);
  display: flex;
  align-items: center;
  gap: 10px;
  animation: slideIn 0.3s cubic-bezier(0.16,1,0.3,1) both;
  max-width: 320px;
}
.toast.success { border-left: 3px solid var(--vue); }
.toast.warning { border-left: 3px solid var(--accent); }

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

/* ═══════════════════════════════════════════
   SIDEBAR BACKDROP (mobile)
═══════════════════════════════════════════ */
.sidebar-backdrop {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.5);
  z-index: 99;
  backdrop-filter: blur(2px);
}
.sidebar-backdrop.show { display: block; }

/* ═══════════════════════════════════════════
   ANIMATIONS
═══════════════════════════════════════════ */
@keyframes fadeUp {
  from { opacity: 0; transform: translateY(24px); }
  to { opacity: 1; transform: translateY(0); }
}

/* ═══════════════════════════════════════════
   SCROLLBAR
═══════════════════════════════════════════ */
::-webkit-scrollbar { width: 6px; height: 6px; }
::-webkit-scrollbar-track { background: transparent; }
::-webkit-scrollbar-thumb { background: var(--border-2); border-radius: 99px; }
::-webkit-scrollbar-thumb:hover { background: var(--text-3); }

/* ═══════════════════════════════════════════
   MINDSET DIAGRAM
═══════════════════════════════════════════ */
.mindset-diagram {
  display: grid;
  grid-template-columns: 1fr auto 1fr;
  gap: 16px;
  align-items: center;
  margin: 24px 0;
}
.mindset-box {
  border-radius: var(--radius);
  padding: 16px 18px;
}
.mindset-box.react { background: var(--react-bg); border: 1px solid var(--react-border); }
.mindset-box.vue { background: var(--vue-bg); border: 1px solid var(--vue-border); }
.mindset-box h4 { font-weight: 700; font-size: 0.82rem; margin-bottom: 8px; }
.mindset-box.react h4 { color: var(--react); }
.mindset-box.vue h4 { color: var(--vue); }
.mindset-box ul { list-style: none; padding: 0; }
.mindset-box ul li {
  font-size: 0.8rem;
  color: var(--text-2);
  padding: 3px 0;
  display: flex;
  gap: 6px;
  align-items: flex-start;
}
.mindset-box ul li::before { content: '·'; flex-shrink: 0; }
.mindset-arrow { text-align: center; font-size: 1.5rem; color: var(--text-3); }

/* ═══════════════════════════════════════════
   RESPONSIVE
═══════════════════════════════════════════ */
@media (max-width: 768px) {
  .sidebar {
    transform: translateX(-100%);
  }
  .sidebar.open {
    transform: translateX(0);
    box-shadow: var(--shadow);
  }
  .sidebar-toggle { display: flex; }
  .sidebar-footer { width: var(--sidebar-w); }

  .main-content { margin-left: 0; }

  .code-panels { grid-template-columns: 1fr; }
  .code-comparison-header { grid-template-columns: 1fr; }
  .concept-grid { grid-template-columns: 1fr; }
  .mindset-diagram { grid-template-columns: 1fr; }
  .mindset-arrow { transform: rotate(90deg); }

  .lesson-topbar { padding: 10px 16px; }
  .lesson-body { padding: 24px 16px; }
  .lesson-nav-footer { padding: 16px; }

  .hero-title { font-size: 2.4rem; }
  .hero-stats { gap: 20px; }

  .challenge-screen { padding: 24px 16px; }
}

@media (max-width: 480px) {
  .lesson-topbar { flex-wrap: wrap; }
  .lesson-breadcrumb { order: 3; width: 100%; }
  .code-tab { font-size: 0.65rem; }
  pre { font-size: 0.72rem; padding: 14px; }
}

/* ═══════════════════════════════════════════
   SINGLE CODE BLOCK
═══════════════════════════════════════════ */
.code-block {
  border-radius: var(--radius-lg);
  overflow: hidden;
  border: 1px solid var(--border);
  margin: 16px 0;
}
.code-block-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 8px 14px;
  background: var(--surface-2);
  border-bottom: 1px solid var(--border);
}
.code-block-lang {
  font-size: 0.7rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--text-3);
}
.code-block pre { background: var(--code-bg); border-radius: 0; margin: 0; }

/* ═══════════════════════════════════════════
   MIGRATION NOTE
═══════════════════════════════════════════ */
.migration-note {
  display: flex;
  gap: 12px;
  background: rgba(97,218,251,0.06);
  border: 1px solid rgba(97,218,251,0.2);
  border-radius: var(--radius);
  padding: 14px 18px;
  margin: 20px 0;
}
.migration-note-icon { font-size: 1rem; flex-shrink: 0; }
.migration-note-body { font-size: 0.84rem; color: var(--text-2); line-height: 1.65; }
.migration-note-body strong { color: var(--react); }

/* ═══════════════════════════════════════════
   MOBILE TOPBAR (hamburger for mobile)
═══════════════════════════════════════════ */
.mobile-topbar {
  display: none;
  align-items: center;
  gap: 12px;
  padding: 14px 16px;
  background: var(--bg-2);
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 60;
}
.mobile-hamburger {
  background: none;
  border: none;
  color: var(--text);
  font-size: 1.3rem;
  cursor: pointer;
}
.mobile-logo {
  font-family: var(--font-display);
  font-weight: 700;
  font-size: 0.9rem;
}

@media (max-width: 768px) {
  .mobile-topbar { display: flex; }
}

/* ═══════════════════════════════════════════
   ADDITIONAL STYLES — appended
═══════════════════════════════════════════ */

/* Keyboard shortcut hint */
.kb-hint {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  font-size: 0.7rem;
  color: var(--text-3);
  margin-top: 8px;
}
kbd {
  background: var(--surface-3);
  border: 1px solid var(--border-2);
  border-radius: 4px;
  padding: 2px 6px;
  font-family: var(--font-code);
  font-size: 0.65rem;
  color: var(--text-2);
}

/* Lesson body - fade-in animation on navigate */
.lesson-body {
  animation: fadeUp 0.35s cubic-bezier(0.16,1,0.3,1) both;
}

/* Enhanced code block line highlight */
pre .highlight-line {
  background: rgba(66,211,146,0.1);
  display: block;
  margin: 0 -20px;
  padding: 0 20px;
  border-left: 2px solid var(--vue);
}

/* Diff table zebra */
.diff-table tr:nth-child(even) td {
  background: var(--surface-2);
}
.diff-table tr:nth-child(even):hover td {
  background: var(--surface-3);
}

/* Hero animated entrance */
@keyframes heroFloat {
  0%, 100% { transform: translateY(0); }
  50% { transform: translateY(-8px); }
}
.hero-title { animation: heroFloat 6s ease-in-out infinite; }

/* Nav item hover effect — left bar slide */
.nav-item::before {
  content: '';
  position: absolute;
  left: 0; top: 0; bottom: 0;
  width: 2px;
  background: var(--vue);
  transform: scaleY(0);
  transition: transform 0.2s ease;
  transform-origin: center;
}
.nav-item:hover::before { transform: scaleY(1); }
.nav-item.active::before { transform: scaleY(1); }

/* Section title with line */
.nav-section-title {
  display: flex;
  align-items: center;
  gap: 6px;
}
.nav-section-title::after {
  content: '';
  flex: 1;
  height: 1px;
  background: var(--border);
  margin-right: 14px;
}

/* Scrollable code panels */
.code-panel pre, .code-block pre {
  max-height: 420px;
  overflow-y: auto;
}

/* Badge for quiz icon */
.quiz-icon { animation: none; }
.quiz-block:hover { border-color: var(--border-2); }
.quiz-block { transition: border-color 0.2s; }

/* Challenge step reveal animation */
.challenge-step-body {
  animation: fadeUp 0.25s ease both;
}

/* Code copy btn always visible on touch devices */
@media (hover: none) {
  .code-copy-btn { opacity: 1 !important; }
}

/* Mobile topbar padding compensation */
@media (max-width: 768px) {
  .hero-screen { padding-top: 56px; }
}

/* Smooth sidebar search clear */
#sidebarSearch::placeholder { color: var(--text-3); }
#sidebarSearch:focus { outline: none; }

/* Status dot on nav items */
.nav-item.completed .nav-icon {
  filter: saturate(0) brightness(1.5);
}
.nav-item.completed .nav-icon::after {
  content: '';
}

/* Progress bar shimmer */
@keyframes shimmer {
  0% { background-position: -200% 0; }
  100% { background-position: 200% 0; }
}
.progress-bar-inner {
  background: linear-gradient(
    90deg,
    var(--vue) 0%,
    var(--react) 50%,
    var(--vue) 100%
  );
  background-size: 200% 100%;
  animation: shimmer 3s linear infinite;
}

/* Lesson nav footer responsive */
@media (max-width: 480px) {
  .lesson-nav-footer { padding: 16px; gap: 8px; }
  .lesson-nav-btn { padding: 8px 14px; font-size: 0.8rem; }
  .back-btn { display: none; }
}

/* Challenge complete banner */
.challenge-complete-banner {
  background: linear-gradient(135deg, rgba(66,211,146,0.1), rgba(97,218,251,0.1));
  border: 1px solid rgba(66,211,146,0.3);
  border-radius: var(--radius-lg);
  padding: 32px;
  text-align: center;
  margin-top: 32px;
}

/* Callout code inline */
.callout code {
  font-family: var(--font-code);
  font-size: 0.8em;
  background: var(--surface-3);
  padding: 2px 6px;
  border-radius: 4px;
  color: var(--vue);
}

/* Pitfall item hover */
.pitfall-item {
  transition: transform 0.15s ease, box-shadow 0.15s ease;
}
.pitfall-item:hover {
  transform: translateX(4px);
  box-shadow: -3px 0 0 var(--danger);
}

/* Smooth screen transitions */
.hero-screen, .lesson-area, .challenge-screen, .cert-screen {
  transition: opacity 0.2s ease;
}

/* Diff table code cells */
.diff-table .react-col, .diff-table .vue-col {
  font-size: 0.75rem;
  line-height: 1.5;
}

/* Expandable open/close transition */
.expandable-body {
  animation: fadeUp 0.2s ease both;
}