:root {
  --ink: #111827;
  --paper: #f7f4ef;
  --panel: #ffffff;
  --line: #d8d2c5;
  --muted: #64748b;
  --blue: #2563eb;
  --green: #059669;
  --red: #dc2626;
  --yellow: #f59e0b;
  --violet: #7c3aed;
  --code: #101624;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  min-height: 100vh;
  color: var(--ink);
  background:
    linear-gradient(90deg, rgba(37, 99, 235, 0.08), transparent 32%),
    linear-gradient(180deg, #fbfaf7 0%, var(--paper) 100%);
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
}

button {
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--panel);
  color: var(--ink);
  cursor: pointer;
  font: inherit;
  font-weight: 700;
  min-height: 40px;
  padding: 0 14px;
}

button:hover {
  border-color: var(--blue);
}

.lab-shell {
  display: grid;
  grid-template-columns: 320px minmax(0, 1fr);
  min-height: 100vh;
}

.lesson-panel {
  border-right: 1px solid var(--line);
  background: rgba(255, 255, 255, 0.78);
  backdrop-filter: blur(10px);
  padding: 22px;
  overflow-y: auto;
}

.brand {
  display: flex;
  gap: 12px;
  align-items: center;
  margin-bottom: 24px;
}

.brand-mark {
  width: 42px;
  height: 42px;
  border-radius: 8px;
  background:
    linear-gradient(135deg, var(--blue) 0 45%, transparent 45%),
    linear-gradient(45deg, var(--green) 0 48%, var(--yellow) 48% 72%, var(--red) 72%);
  box-shadow: 0 8px 22px rgba(17, 24, 39, 0.16);
  flex: 0 0 auto;
}

.brand h1 {
  font-size: 1.1rem;
  margin: 0;
}

.brand p {
  color: var(--muted);
  margin: 2px 0 0;
  font-size: 0.88rem;
}

.lesson-list {
  display: grid;
  gap: 8px;
}

.lesson-button {
  display: grid;
  grid-template-columns: 38px minmax(0, 1fr);
  gap: 10px;
  align-items: center;
  width: 100%;
  border-color: transparent;
  text-align: left;
  padding: 10px;
}

.lesson-button span:first-child {
  display: grid;
  place-items: center;
  width: 34px;
  height: 34px;
  border-radius: 8px;
  background: #edf2ff;
  color: var(--blue);
  font-weight: 800;
}

.lesson-button strong {
  display: block;
  font-size: 0.92rem;
  overflow-wrap: anywhere;
}

.lesson-button small {
  display: block;
  color: var(--muted);
  font-size: 0.78rem;
  margin-top: 2px;
}

.lesson-button.active {
  border-color: var(--blue);
  background: #eef5ff;
}

.resource-links {
  display: grid;
  gap: 8px;
  margin-top: 22px;
  padding-top: 18px;
  border-top: 1px solid var(--line);
}

.resource-links a {
  display: block;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: white;
  color: var(--ink);
  font-weight: 800;
  padding: 10px 12px;
  text-decoration: none;
}

.resource-links a:hover {
  border-color: var(--blue);
  color: var(--blue);
}

.workspace {
  display: grid;
  grid-template-rows: auto minmax(0, 1fr);
  min-width: 0;
}

.lesson-header {
  display: flex;
  gap: 18px;
  justify-content: space-between;
  align-items: end;
  padding: 22px 26px;
  border-bottom: 1px solid var(--line);
  background: rgba(255, 255, 255, 0.68);
}

.eyebrow {
  margin: 0 0 6px;
  color: var(--violet);
  font-size: 0.78rem;
  font-weight: 800;
  text-transform: uppercase;
}

.lesson-header h2 {
  margin: 0;
  font-size: clamp(1.35rem, 2vw, 2rem);
}

.goal {
  max-width: 780px;
  color: var(--muted);
  margin: 8px 0 0;
  line-height: 1.45;
}

.actions {
  display: flex;
  gap: 8px;
  flex-wrap: wrap;
  justify-content: flex-end;
}

#runButton {
  background: var(--blue);
  border-color: var(--blue);
  color: white;
}

.activity {
  display: grid;
  grid-template-columns: minmax(360px, 0.95fr) minmax(360px, 1.05fr);
  gap: 0;
  min-height: 0;
}

.editor-pane,
.preview-pane {
  min-width: 0;
  min-height: 0;
}

.editor-pane {
  border-right: 1px solid var(--line);
  background: var(--code);
}

#codeEditor {
  display: block;
  width: 100%;
  height: 100%;
  min-height: 520px;
  resize: none;
  border: 0;
  outline: 0;
  padding: 20px;
  background: var(--code);
  color: #e5e7eb;
  font: 15px/1.55 "JetBrains Mono", "SFMono-Regular", Consolas, monospace;
  tab-size: 2;
}

.preview-pane {
  background: #172033;
}

#previewFrame {
  display: block;
  width: 100%;
  height: 100%;
  min-height: 520px;
  border: 0;
  background: #172033;
}

@media (max-width: 980px) {
  .lab-shell {
    grid-template-columns: 1fr;
  }

  .lesson-panel {
    border-right: 0;
    border-bottom: 1px solid var(--line);
    max-height: 300px;
  }

  .activity {
    grid-template-columns: 1fr;
  }

  .editor-pane {
    border-right: 0;
    border-bottom: 1px solid var(--line);
  }
}

@media (max-width: 640px) {
  .lesson-header {
    align-items: stretch;
    flex-direction: column;
    padding: 18px;
  }

  .actions {
    justify-content: stretch;
  }

  .actions button {
    flex: 1 1 120px;
  }
}