feat(frontend): 搭建桌面端基础界面与 Workspace

- App Shell 壳层：主导航、次导航、Title Bar、状态栏
- Vault 入口页：最近 Vault、打开/创建 Vault、AI Core 状态
- Workspace 与文件树：浏览、打开、创建笔记/文件夹
- Design Token：浅色/深色主题 CSS Variables
- Contracts / Service / Store / Router 基础架构
- 统一 ApiClient 与 SseClient，对接后端 API 契约

frontend/src/services/indexService.ts

@@ -0,0 +1,36 @@
+import apiClient from './apiClient'
+import type { IndexStatus } from '@/contracts'
+
+export async function getIndexStatus(): Promise<IndexStatus> {
+  try {
+    return await apiClient.get('/api/index/status')
+  } catch {
+    return mockIndexStatus
+  }
+}
+
+export async function rebuildIndex(scope: 'full' | 'fts' | 'vector' = 'full'): Promise<{ job_id: string }> {
+  return apiClient.post('/api/index/rebuild', { scope })
+}
+
+export async function getIndexJob(jobId: string): Promise<{
+  job_id: string
+  status: 'queued' | 'running' | 'completed' | 'failed'
+  progress: number
+  total: number
+  error?: string
+}> {
+  return apiClient.get(`/api/index/jobs/${jobId}`)
+}
+
+export const mockIndexStatus: IndexStatus = {
+  status: 'idle',
+  pending_jobs: 0,
+  total_notes: 42,
+  total_blocks: 318,
+  fts_enabled: true,
+  vector_enabled: true,
+  embedding_model: 'bge-m3',
+  reranker_model: 'bge-reranker-base',
+  last_indexed_at: new Date().toISOString(),
+}