/* Theme */
:root {
  --bg: #0b1020;
  --card: #0e1528;
  --text: #eaf2ff;
  --muted: #9fb2d0;
  --accent: #4c9cff;
  --accent-2: #22d3ee;
  --border: rgba(255,255,255,.08);
}
* { box-sizing: border-box }
html, body { height: 100% }
body {
  margin: 0;
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, Segoe UI, Roboto, Helvetica, Arial;
  color: var(--text);
  background:
    radial-gradient(1200px 500px at -10% -20%, rgba(76,156,255,.15), transparent 60%),
    radial-gradient(1200px 500px at 120% 0%, rgba(34,211,238,.12), transparent 60%),
    linear-gradient(180deg,#091026,#070d1e 60%,#060a17);
}

/* Layout */
.layout { display: grid; grid-template-columns: 310px 1fr; gap: 0; width: 98%; margin: 0 auto }
@media (max-width: 1020px) { .layout { grid-template-columns: 1fr } .sidebar { position: static; height: auto } }

.sidebar {
  position: sticky; top: 0; align-self: start; height: 100vh; overflow: auto;
  padding: 16px; border-right: 1px solid var(--border);
  background: linear-gradient(180deg, rgba(11,16,32,.75), rgba(11,16,32,.25));
  backdrop-filter: blur(6px);
}
.brand { display: flex; align-items: center; gap: 10px; text-decoration: none; color: var(--text) }
.logo { border-radius: 8px; box-shadow: 0 6px 24px rgba(76,156,255,.35), inset 0 0 18px rgba(255,255,255,.2) }
.brand .sub { font-size: 12px; color: var(--muted) }
.search-box { margin: 12px 0 }
.search-box input { width: 100%; padding: 10px 12px; border-radius: 10px; border: 1px solid var(--border); background: rgba(0,0,0,.1); color: var(--text) }

.toc { display: grid; gap: 6px }
.toc .group { margin-top: 10px }
.toc h4 { margin: 8px 0 4px; font-size: 13px; color: var(--muted) }
.toc button.link {
  width: 100%; text-align: left; background: transparent; color: var(--text); border: 1px solid var(--border);
  border-radius: 10px; padding: 10px; cursor: pointer;
}
.toc button.link:hover { border-color: rgba(255,255,255,.2) }
.toc .done { border-color: #21d07a; }
.toc .current { border-color: var(--accent); box-shadow: 0 0 0 2px rgba(76,156,255,.2) inset }

/* kept for compatibility even if unused */
.sidebar-actions { display:flex; gap: 8px; flex-wrap: wrap; margin-top: 12px }
.btn { background: var(--accent); color: #fff; border: none; padding: 10px 12px; border-radius: 10px; cursor: pointer; font-weight: 800 }
.btn:hover { background: var(--accent-2) }
.btn.ghost { background: transparent; color: var(--muted); border: 1px solid var(--border) }
.btn.small { font-size: 12px; padding: 8px 10px }

.side-footer { color: var(--muted); font-size: 12px; padding-top: 12px }

.content { padding: 16px }
.topbar { display: flex; align-items: center; justify-content: space-between; gap: 12px; margin-bottom: 12px }
.crumbs { color: var(--muted) }

.card { background: linear-gradient(180deg,rgba(255,255,255,.04),rgba(255,255,255,.02)); border: 1px solid var(--border); padding: 16px; border-radius: 14px; box-shadow: 0 6px 20px rgba(0,0,0,.35) }
.lesson-head h1 { margin: 0 0 6px }
.lesson-body { line-height: 1.6 }
.lesson-body img, .lesson-body iframe { max-width: 100%; border-radius: 10px; border: 1px solid var(--border) }
.lesson-body pre, .lesson-body code { background: rgba(255,255,255,.06); padding: 2px 6px; border-radius: 6px }
.lesson-nav { display: flex; gap: 8px; justify-content: flex-end; margin-top: 12px }

.info { margin-top: 12px }
.muted { color: var(--muted) }

/* ====================
   Mobile & small screens
   ==================== */
@media (max-width: 960px) {
  .layout {
    display: flex;
    flex-direction: column;
    min-height: 100dvh;
    width: 100%;
  }
  .sidebar {
    position: static;
    width: 100%;
    max-height: none;
    height: auto;
    border-right: none;
    border-bottom: 1px solid var(--border);
    padding: 12px 12px 8px;
    backdrop-filter: none;
  }
  .brand { gap: 10px; }
  .search-box { margin: 10px 0; }
  .toc {
    max-height: 50vh; /* prevent overlap with content */
    overflow: auto;
    -webkit-overflow-scrolling: touch;
  }
  .content {
    padding: 12px;
  }
  .topbar {
    position: sticky;
    top: 0;
    z-index: 10;
    background: var(--bg);
    padding: 10px 0;
    gap: 8px;
    flex-wrap: wrap; /* avoid overlapping actions */
    margin-bottom: 10px;
  }
  .top-actions {
    display: flex;
    gap: 8px;
    flex-wrap: wrap;
    width: 100%;
  }
  .top-actions .btn,
  .top-actions .btn.ghost {
    flex: 1 1 180px; /* grow and wrap nicely */
  }
  .lesson {
    margin-top: 12px;
  }
  .lesson-head h1 {
    font-size: 1.25rem;
    line-height: 1.3;
  }
  .lesson-body {
    font-size: 0.98rem;
  }
  .lesson-body table {
    display: block;
    overflow-x: auto;
    border-spacing: 0;
    -webkit-overflow-scrolling: touch;
  }
  .lesson-body pre {
    white-space: pre-wrap;       /* wrap long code to avoid overflow */
    word-break: break-word;
  }
  .lesson-nav {
    justify-content: space-between;
    gap: 10px;
  }
  #prevBtn, #nextBtn {
    flex: 1 1 48%;
    min-height: 44px; /* touch target */
  }
}

/* Extra small phones */
@media (max-width: 480px) {
  .brand strong { font-size: 1rem; }
  .brand .sub { font-size: 0.8rem; }
  .search-box input { font-size: 16px; } /* prevent iOS zoom */
}

.side-footer {
  color: var(--muted);
  font-size: 12px;
  padding-top: 12px;
  margin-top: 8px;
  border-top: 1px solid var(--border);
  text-align: center;
  text-decoration: none;
}
.side-footer a:link {
  text-decoration: none;
  color: #22d3ee;
}
@media (max-width: 960px) {
  .side-footer { padding-bottom: 8px; }
}