* { margin:0; padding:0; box-sizing:border-box; }
body {
  font-family: 'Inter', sans-serif;
  background: #f4f6f9;
  height: 100vh;
  overflow: hidden;
}

.app {
  display: flex;
  height: 100vh;
}

.sidebar {
  width: 380px;
  background: #2c3e50;
  color: white;
  overflow-y: auto;
  border-right: 1px solid #1a252f;
}

.sidebar-header {
  padding: 20px;
  background: #1a252f;
  display: flex;
  justify-content: space-between;
  align-items: center;
  font-size: 18px;
}

#tree {
  padding: 10px 0;
}

.tree-node {
  padding: 8px 20px;
  cursor: pointer;
  user-select: none;
  transition: 0.2s;
  position: relative;
}

.tree-node:hover { background: rgba(255,255,255,0.1); }
.tree-node.selected { background: #3498db; font-weight: 600; }

.tree-node.lesson { font-weight: 600; font-size: 15px; }
.tree-node.topic { padding-left: 40px; font-size: 14px; color: #bdc3c7; }
.tree-node.question { padding-left: 60px; font-size: 13px; color: #95a5a6; }

.children {
  display: none;
  overflow: hidden;
}

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

.topic > .children {
  padding-left: 20px;
}

.tree-node::before {
  content: "▶";
  position: absolute;
  left: 8px;
  font-size: 10px;
  transition: 0.2s;
}
.tree-node.open::before { transform: rotate(90deg); }

.content {
  flex: 1;
  padding: 20px;
  overflow-y: auto;
  background: #fff;
}

.toolbar {
  padding: 15px 0;
  border-bottom: 1px solid #eee;
  margin-bottom: 20px;
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
}

.welcome {
  text-align: center;
  margin-top: 100px;
  color: #7f8c8d;
}

.btn {
  padding: 10px 16px;
  border: none;
  border-radius: 6px;
  cursor: pointer;
  font-size: 14px;
  font-weight: 500;
}
.btn.success { background: #27ae60; color: white; }
.btn.warning { background: #f39c12; color: white; }
.btn.danger { background: #e74c3c; color: white; }
.btn.small { padding: 6px 12px; font-size: 13px; }

input[type="file"] { display: none; }

.editor {
  background: #f8f9fa;
  padding: 20px;
  border-radius: 10px;
  border: 1px solid #ddd;
}

.editor h3 { margin: 0 0 15px; color: #2c3e50; }
.editor input, .editor textarea {
  width: 100%;
  padding: 12px;
  margin: 10px 0;
  border: 1px solid #ddd;
  border-radius: 6px;
  font-family: inherit;
}
.editor textarea { min-height: 120px; resize: vertical; }