:root {
  --bg: #f8fafc;
  --card-bg: #ffffff;
  --text: #0f172a;
  --text-muted: #64748b;
  --border: #e2e8f0;
  --accent: #4f46e5;
  --accent-soft: #eef2ff;
  --radius: 12px;
  --shadow: 0 1px 3px rgba(15, 23, 42, 0.08), 0 4px 12px rgba(15, 23, 42, 0.05);
  --shadow-hover: 0 4px 8px rgba(15, 23, 42, 0.1), 0 12px 24px rgba(15, 23, 42, 0.1);
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.5;
}

.container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 24px;
}

/* Header — sticky bar matching the MathVibe Gallery header */
.site-header {
  position: sticky;
  top: 0;
  z-index: 50;
  background: rgba(255, 255, 255, 0.95);
  backdrop-filter: blur(8px);
  -webkit-backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--border);
}

.header-bar {
  height: 64px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
}

.brand {
  display: flex;
  align-items: center;
  gap: 8px;
  text-decoration: none;
}

.brand img {
  height: 44px;
  width: auto;
  object-fit: contain;
}

.brand-label {
  font-size: 0.75rem;
  color: var(--text-muted);
}

.header-nav a {
  font-size: 0.875rem;
  font-weight: 500;
  color: var(--text-muted);
  text-decoration: none;
  padding: 6px 12px;
  border-radius: 8px;
}

.header-nav a:hover {
  color: var(--text);
  background: var(--bg);
}

/* Hero */
.hero {
  background: var(--card-bg);
  border-bottom: 1px solid var(--border);
  padding: 40px 0 0;
}

.hero h1 {
  margin: 0 0 8px;
  font-size: 1.9rem;
  letter-spacing: -0.02em;
}

.subtitle {
  margin: 0;
  max-width: 720px;
  color: var(--text-muted);
}

/* Filter tabs */
.filters {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  padding: 20px 0 16px;
}

.filter-btn {
  border: 1px solid var(--border);
  background: var(--card-bg);
  color: var(--text-muted);
  padding: 6px 14px;
  border-radius: 999px;
  font-size: 0.875rem;
  cursor: pointer;
  transition: all 0.15s ease;
}

.filter-btn:hover { border-color: var(--accent); color: var(--accent); }

.filter-btn.active {
  background: var(--accent);
  border-color: var(--accent);
  color: #fff;
}

/* Sections */
.group-section { padding-top: 36px; }

.group-section h2 {
  margin: 0 0 4px;
  font-size: 1.25rem;
  letter-spacing: -0.01em;
}

.group-desc {
  margin: 0 0 20px;
  color: var(--text-muted);
  font-size: 0.925rem;
}

/* Card grid */
.grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
  gap: 24px;
}

.card {
  background: var(--card-bg);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  overflow: hidden;
  box-shadow: var(--shadow);
  transition: box-shadow 0.15s ease, transform 0.15s ease;
  display: flex;
  flex-direction: column;
}

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

/* Live scaled-down iframe preview of the lesson */
.preview {
  position: relative;
  aspect-ratio: 16 / 10;
  background: linear-gradient(135deg, #eef2ff 0%, #f5f3ff 100%);
  overflow: hidden;
  display: block;
}

.preview iframe {
  position: absolute;
  top: 0;
  left: 0;
  /* 20px wider than the 1280px scale basis so the page's own scrollbar is
     clipped off the right edge of the card. */
  width: 1300px;
  height: 800px;
  border: 0;
  transform-origin: top left;
  pointer-events: none; /* card click opens the lesson; no scrolling inside the thumbnail */
  background: #fff;
}

.preview iframe {
  z-index: 1;
}

.preview .placeholder {
  position: absolute;
  inset: 0;
  z-index: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--accent);
  font-size: 0.875rem;
}

/* Clickable overlay so the whole preview opens the lesson */
.preview .overlay {
  position: absolute;
  inset: 0;
  z-index: 2;
}

.card-body {
  padding: 16px;
  display: flex;
  flex-direction: column;
  gap: 4px;
  flex: 1;
}

.card-topic {
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--accent);
  font-weight: 600;
}

.card-title {
  margin: 0;
  font-size: 1.05rem;
  font-weight: 600;
}

.card-title a { color: inherit; text-decoration: none; }
.card-title a:hover { color: var(--accent); }

.card-notes {
  margin: 0;
  font-size: 0.875rem;
  color: var(--text-muted);
}

.card-footer {
  margin-top: auto;
  padding-top: 12px;
}

.open-link {
  display: inline-block;
  font-size: 0.875rem;
  font-weight: 500;
  color: var(--accent);
  background: var(--accent-soft);
  padding: 6px 14px;
  border-radius: 8px;
  text-decoration: none;
  transition: background 0.15s ease;
}

.open-link:hover { background: #e0e7ff; }

.empty-state {
  padding: 80px 0;
  text-align: center;
  color: var(--text-muted);
}

/* Footer */
.site-footer {
  margin-top: 64px;
  background: var(--card-bg);
  border-top: 1px solid var(--border);
  padding: 24px 0;
  color: var(--text-muted);
  font-size: 0.875rem;
}

.footer-bar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  flex-wrap: wrap;
}

.footer-bar .brand img { height: 32px; }
.footer-bar p { margin: 0; }

@media (max-width: 640px) {
  .grid { grid-template-columns: 1fr; }
  .hero h1 { font-size: 1.5rem; }
  .brand-label { display: none; }
}