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/main.ts

@@ -1,5 +1,17 @@
 import { createApp } from 'vue'
+import { createPinia } from 'pinia'
 import App from './App.vue'
-import './style.css'
+import router from './router'
+import './styles/tokens.css'
+import { useThemeStore } from './stores/theme'
 
-createApp(App).mount('#app')
+const app = createApp(App)
+const pinia = createPinia()
+
+app.use(pinia)
+app.use(router)
+
+const themeStore = useThemeStore()
+themeStore.initTheme()
+
+app.mount('#app')