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/stores/index.ts

@@ -0,0 +1,11 @@
+export { useWorkspaceStore } from './workspace'
+export { useEditorStore } from './editor'
+export { useSearchStore } from './search'
+export { useChatStore } from './chat'
+export { useAgentStore } from './agent'
+export { useSkillStore } from './skill'
+export { usePluginStore } from './plugin'
+export { useTaskStore } from './task'
+export { useThemeStore } from './theme'
+export { useProviderStore } from './provider'
+export { useSettingsStore } from './settings'