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/systemService.ts

@@ -0,0 +1,23 @@
+import apiClient from './apiClient'
+import type { SystemStatus } from '@/contracts'
+
+export async function healthCheck(): Promise<{ status: string }> {
+  try {
+    return await apiClient.get<{ status: string }>('/health')
+  } catch {
+    return { status: 'unavailable' }
+  }
+}
+
+export async function getStatus(): Promise<SystemStatus> {
+  try {
+    return await apiClient.get<SystemStatus>('/api/status')
+  } catch {
+    return {
+      name: 'notes-agent',
+      version: '0.1.0',
+      environment: import.meta.env.DEV ? 'development' : 'production',
+      ai_core_available: false,
+    }
+  }
+}