feat: add session tree search functionality

- Add useTreeSearch composable with filtering logic
- Implement search bar with Ctrl+F shortcut
- Show search results with ancestor paths
- Add search match indicator (cyan dot)
- Display "no results" message when search is empty
- Support clear search with X button

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/SessionTreeNode.vue

@@ -32,6 +32,7 @@ const connection = computed(() => {
 
 // Drag-drop functionality
 const dragHandlers = inject<any>('dragHandlers', null)
+const isSearchMatch = inject<(nodeId: string) => boolean>('isSearchMatch', () => false)
 
 const isDragging = computed(() =>
   dragHandlers?.draggedNode.value?.id === props.node.id
@@ -138,8 +139,15 @@ function handleDragEnd() {
 
       <span class="text-sm truncate flex-1">{{ node.name }}</span>
 
+      <!-- Search match indicator -->
       <span
-        v-if="connection"
+        v-if="isSearchMatch(node.id)"
+        class="w-1.5 h-1.5 rounded-full bg-cyan-400 flex-shrink-0"
+        title="搜索匹配"
+      />
+
+      <span
+        v-else-if="connection"
         class="w-2 h-2 rounded-full bg-slate-600"
       />
     </div>