:root {
  --border: #e6e6e6;
  --text: #111;
  --muted: #666;
  --bg: #fff;
  --bg2: #fafafa;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: Arial, Helvetica, sans-serif;
  color: var(--text);
  background: var(--bg);
}

.layout {
  display: grid;
  grid-template-columns: 320px 1fr;
  height: 100vh;
}

/* Sidebar */
.sidebar {
  border-right: 1px solid var(--border);
  background: var(--bg2);
  overflow: auto;
}

.sidebar-header {
  position: sticky;
  top: 0;
  background: var(--bg2);
  padding: 14px;
  border-bottom: 1px solid var(--border);
  display: grid;
  gap: 10px;
}

.title {
  font-weight: 700;
  font-size: 16px;
}

.search {
  width: 100%;
  padding: 10px;
  border: 1px solid var(--border);
  border-radius: 8px;
  outline: none;
  background: #fff;
}

.clear {
  width: 100%;
  padding: 10px;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: #fff;
  cursor: pointer;
}

.tree {
  padding: 8px 10px 20px 10px;
}

/* Accordion nodes */
.node {
  margin: 4px 0;
  border-radius: 8px;
}

.node-header {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 8px 10px;
  cursor: pointer;
  user-select: none;
  border-radius: 8px;
}

.node-header:hover {
  background: #f0f0f0;
}

.caret {
  width: 18px;
  text-align: center;
  color: var(--muted);
}

.label {
  font-weight: 600;
  font-size: 13px;
}

.children {
  margin-left: 18px;
  padding-left: 6px;
  border-left: 1px dashed var(--border);
  display: none;
}

.node.open > .children {
  display: block;
}

/* Lesson items */
.item {
  padding: 7px 10px;
  margin: 2px 0;
  border-radius: 8px;
  cursor: pointer;
  font-size: 13px;
}

.item:hover {
  background: #f0f0f0;
}

.item.active {
  background: #eaeaea;
}

/* Main */
.main {
  padding: 22px 22px 18px 22px;
  overflow: hidden;
  display: grid;
  grid-template-rows: auto auto 1fr auto auto;
  gap: 12px;
}

.breadcrumb {
  font-size: 12px;
  color: var(--muted);
}

.main-top {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
}

.lesson-title {
  margin: 0;
  font-size: 22px;
}

.open-btn {
  display: inline-block;
  padding: 10px 14px;
  border: 1px solid var(--border);
  border-radius: 10px;
  text-decoration: none;
  color: var(--text);
  background: #fff;
  white-space: nowrap;
}

.open-btn.disabled {
  opacity: 0.45;
  pointer-events: none;
}

/* Viewer */
.viewer-wrap {
  border: 1px solid var(--border);
  border-radius: 12px;
  background: #fff;
  overflow: hidden;
  height: 100%;
  min-height: 420px;
}

.viewer {
  width: 100%;
  height: 100%;
  border: 0;
}

/* Messages */
.hint, .error {
  padding: 12px 14px;
  border: 1px solid var(--border);
  border-radius: 12px;
  background: #fff;
  color: var(--muted);
  max-width: 900px;
}

.error {
  color: #7a1c1c;
}