/* ---------- design tokens ---------- */
:root {
  --bg: #0f1218;
  --bg-elev: #161b24;
  --bg-elev-2: #1d2431;
  --border: #2a3142;
  --fg: #e6edf3;
  --fg-dim: #8b95a7;
  --accent: #7c6cff;   /* C# purple */
  --accent-dim: #5a4dcc;
  --green: #4ade80;
  --red: #f87171;
  --yellow: #facc15;
  --blue: #60a5fa;
  --mono: "JetBrains Mono", "Fira Code", ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  --sans: -apple-system, BlinkMacSystemFont, "Segoe UI", system-ui, sans-serif;
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  height: 100%;
  background: var(--bg);
  color: var(--fg);
  font-family: var(--sans);
  font-size: 15px;
  line-height: 1.55;
}

/* ---------- header ---------- */
.topbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 14px 24px;
  background: var(--bg-elev);
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 10;
}

.brand {
  display: flex;
  align-items: center;
  gap: 14px;
}

.logo {
  width: 42px;
  height: 42px;
  display: grid;
  place-items: center;
  background: linear-gradient(135deg, var(--accent), var(--accent-dim));
  color: white;
  font-weight: 700;
  font-size: 18px;
  border-radius: 10px;
  letter-spacing: -0.5px;
}

.brand h1 {
  margin: 0;
  font-size: 17px;
  font-weight: 600;
}

.tagline {
  margin: 0;
  color: var(--fg-dim);
  font-size: 12px;
}

.progress {
  display: flex;
  align-items: center;
  gap: 12px;
  font-size: 13px;
  color: var(--fg-dim);
}

.progress-bar {
  width: 140px;
  height: 8px;
  background: var(--bg-elev-2);
  border-radius: 999px;
  overflow: hidden;
}

#progress-fill {
  height: 100%;
  width: 0%;
  background: linear-gradient(90deg, var(--accent), var(--green));
  transition: width 0.3s ease;
}

/* ---------- layout ---------- */
.layout {
  display: grid;
  grid-template-columns: 280px 1fr;
  min-height: calc(100vh - 71px);
}

.sidebar {
  background: var(--bg-elev);
  border-right: 1px solid var(--border);
  padding: 16px 8px;
  overflow-y: auto;
  max-height: calc(100vh - 71px);
  position: sticky;
  top: 71px;
}

.sidebar-loading {
  color: var(--fg-dim);
  padding: 16px;
  font-size: 13px;
}

.category {
  padding: 14px 12px 6px;
  font-size: 11px;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--fg-dim);
  font-weight: 600;
}

.lesson-item {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 8px 12px;
  border-radius: 8px;
  cursor: pointer;
  font-size: 14px;
  color: var(--fg);
  transition: background 0.15s;
  user-select: none;
}

.lesson-item:hover {
  background: var(--bg-elev-2);
}

.lesson-item.active {
  background: var(--accent);
  color: white;
  font-weight: 500;
}

.lesson-item.active .lesson-num { color: rgba(255, 255, 255, 0.7); }
.lesson-item.done .status-dot { background: var(--green); }

.lesson-num {
  font-family: var(--mono);
  font-size: 11px;
  color: var(--fg-dim);
  width: 22px;
}

.status-dot {
  width: 8px;
  height: 8px;
  border-radius: 999px;
  background: var(--border);
  margin-left: auto;
  flex-shrink: 0;
}

.lesson-item.active .status-dot { background: rgba(255, 255, 255, 0.5); }
.lesson-item.active.done .status-dot { background: white; }

/* ---------- content ---------- */
.content {
  padding: 32px 40px 80px;
  max-width: 1100px;
}

.empty-state {
  max-width: 580px;
  padding: 40px 0;
}

.empty-state h2 {
  font-size: 28px;
  margin: 0 0 12px;
}

.empty-state p {
  color: var(--fg-dim);
}

.empty-state ul {
  color: var(--fg-dim);
  padding-left: 20px;
}

.empty-state li { margin: 6px 0; }

.lesson-header {
  display: flex;
  align-items: baseline;
  gap: 12px;
  margin-bottom: 6px;
}

.lesson-header .lesson-order {
  font-family: var(--mono);
  font-size: 13px;
  color: var(--accent);
  font-weight: 600;
}

.lesson-header .lesson-category {
  font-size: 11px;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--fg-dim);
  background: var(--bg-elev-2);
  padding: 3px 8px;
  border-radius: 4px;
  margin-left: auto;
}

.lesson-title {
  margin: 0 0 20px;
  font-size: 28px;
  font-weight: 700;
  letter-spacing: -0.3px;
}

/* ---------- callouts ---------- */
.callout {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-left: 3px solid var(--accent);
  border-radius: 8px;
  padding: 14px 18px;
  margin: 20px 0;
}

.callout .label {
  font-size: 11px;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--accent);
  margin-bottom: 6px;
}

.callout.ts {
  border-left-color: var(--blue);
}

.callout.ts .label { color: var(--blue); }

.callout p { margin: 4px 0; }

/* ---------- explanation (markdown-ish) ---------- */
.explanation {
  font-size: 15px;
  line-height: 1.7;
}

.explanation code {
  font-family: var(--mono);
  font-size: 13px;
  background: var(--bg-elev-2);
  padding: 2px 6px;
  border-radius: 4px;
  color: #e2d9ff;
}

.explanation pre {
  background: #0b0e14;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 14px 16px;
  overflow-x: auto;
  font-family: var(--mono);
  font-size: 13px;
  line-height: 1.6;
}

.explanation pre code {
  background: transparent;
  padding: 0;
  color: #e6edf3;
}

/* ---------- editor ---------- */
.editor-section {
  margin-top: 28px;
}

.editor-section h3 {
  font-size: 13px;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--fg-dim);
  margin: 0 0 10px;
}

.editor-wrap {
  background: #0b0e14;
  border: 1px solid var(--border);
  border-radius: 10px;
  overflow: hidden;
}

.editor-toolbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 8px 12px;
  background: var(--bg-elev-2);
  border-bottom: 1px solid var(--border);
}

.editor-toolbar .filename {
  font-family: var(--mono);
  font-size: 12px;
  color: var(--fg-dim);
}

.editor-toolbar .buttons {
  display: flex;
  gap: 8px;
}

button {
  background: var(--bg-elev-2);
  color: var(--fg);
  border: 1px solid var(--border);
  padding: 6px 14px;
  border-radius: 6px;
  font-family: inherit;
  font-size: 13px;
  cursor: pointer;
  transition: all 0.15s;
}

button:hover:not(:disabled) {
  background: var(--border);
}

button:disabled {
  opacity: 0.5;
  cursor: not-allowed;
}

button.primary {
  background: var(--accent);
  border-color: var(--accent);
  color: white;
  font-weight: 500;
}

button.primary:hover:not(:disabled) {
  background: var(--accent-dim);
}

textarea.editor {
  width: 100%;
  min-height: 260px;
  background: transparent;
  color: var(--fg);
  border: none;
  padding: 14px 16px;
  font-family: var(--mono);
  font-size: 13px;
  line-height: 1.6;
  resize: vertical;
  outline: none;
  tab-size: 4;
  -moz-tab-size: 4;
}

.output-panel {
  background: #0b0e14;
  border: 1px solid var(--border);
  border-radius: 10px;
  margin-top: 12px;
  overflow: hidden;
}

.output-header {
  padding: 8px 12px;
  background: var(--bg-elev-2);
  border-bottom: 1px solid var(--border);
  font-size: 12px;
  color: var(--fg-dim);
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.output-body {
  padding: 14px 16px;
  font-family: var(--mono);
  font-size: 13px;
  white-space: pre-wrap;
  min-height: 60px;
  color: var(--fg);
}

.output-body.error { color: var(--red); }
.output-body.ok    { color: var(--green); }
.output-body.idle  { color: var(--fg-dim); font-style: italic; }

/* ---------- key points ---------- */
.key-points {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 16px 20px;
  margin-top: 24px;
}

.key-points h3 {
  margin: 0 0 10px;
  font-size: 13px;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--fg-dim);
}

.key-points ul {
  margin: 0;
  padding-left: 20px;
}

.key-points li {
  margin: 6px 0;
}

.key-points code {
  font-family: var(--mono);
  font-size: 12px;
  background: var(--bg-elev-2);
  padding: 1px 5px;
  border-radius: 3px;
  color: #e2d9ff;
}

/* ---------- quiz ---------- */
.quiz {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 18px 20px;
  margin-top: 20px;
}

.quiz-question {
  font-weight: 500;
  margin-bottom: 12px;
}

.quiz-choices {
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.quiz-choice {
  padding: 10px 14px;
  background: var(--bg-elev-2);
  border: 1px solid var(--border);
  border-radius: 6px;
  cursor: pointer;
  text-align: left;
  font-family: inherit;
  font-size: 14px;
  color: var(--fg);
  transition: all 0.15s;
}

.quiz-choice:hover:not(:disabled) {
  border-color: var(--accent);
}

.quiz-choice.correct {
  background: rgba(74, 222, 128, 0.15);
  border-color: var(--green);
  color: var(--green);
}

.quiz-choice.wrong {
  background: rgba(248, 113, 113, 0.15);
  border-color: var(--red);
  color: var(--red);
}

.quiz-explanation {
  margin-top: 12px;
  padding: 12px 14px;
  background: var(--bg-elev-2);
  border-radius: 6px;
  font-size: 13px;
  color: var(--fg-dim);
}

.quiz-explanation strong { color: var(--fg); }

/* ---------- completion ---------- */
.mark-done {
  margin-top: 28px;
  display: flex;
  gap: 12px;
  align-items: center;
}

.mark-done .done-badge {
  color: var(--green);
  font-size: 13px;
}

/* ---------- scrollbars ---------- */
::-webkit-scrollbar { width: 10px; height: 10px; }
::-webkit-scrollbar-track { background: var(--bg); }
::-webkit-scrollbar-thumb { background: var(--border); border-radius: 5px; }
::-webkit-scrollbar-thumb:hover { background: var(--fg-dim); }

/* ---------- small screens ---------- */
@media (max-width: 820px) {
  .layout { grid-template-columns: 1fr; }
  .sidebar { position: static; max-height: none; border-right: none; border-bottom: 1px solid var(--border); }
  .content { padding: 20px; }
  .topbar { flex-direction: column; align-items: flex-start; gap: 12px; }
}