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/index.ts

@@ -0,0 +1,15 @@
+export { apiClient, ApiErrorClass } from './apiClient'
+export type { ApiError } from './apiClient'
+export { SseClient } from './sseClient'
+export type { SseClientOptions, SseEventHandler } from './sseClient'
+export * as noteService from './noteService'
+export * as searchService from './searchService'
+export * as chatService from './chatService'
+export * as agentService from './agentService'
+export * as skillService from './skillService'
+export * as pluginService from './pluginService'
+export * as providerService from './providerService'
+export * as taskService from './taskService'
+export * as indexService from './indexService'
+export * as systemService from './systemService'
+export * as workspaceService from './workspaceService'