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/noteService.ts

@@ -0,0 +1,38 @@
+import apiClient from './apiClient'
+import type { Note, NoteBlock } from '@/contracts'
+
+export async function listNotes(params?: {
+  folder?: string
+  tag?: string
+  limit?: number
+  offset?: number
+}): Promise<{ items: Note[]; total: number }> {
+  return apiClient.get('/api/notes', { params })
+}
+
+export async function getNote(noteId: string): Promise<{ note: Note; blocks: NoteBlock[] }> {
+  return apiClient.get(`/api/notes/${noteId}`)
+}
+
+export async function createNote(data: {
+  title: string
+  folder_path?: string
+  content?: string
+}): Promise<Note> {
+  return apiClient.post('/api/notes', data)
+}
+
+export async function updateNote(
+  noteId: string,
+  data: { title?: string; content?: string; tags?: string[] }
+): Promise<Note> {
+  return apiClient.patch(`/api/notes/${noteId}`, data)
+}
+
+export async function deleteNote(noteId: string): Promise<void> {
+  return apiClient.delete(`/api/notes/${noteId}`)
+}
+
+export async function moveNote(noteId: string, target_folder: string): Promise<Note> {
+  return apiClient.post(`/api/notes/${noteId}/move`, { target_folder })
+}