feat(frontend): 构建前端 MVP，实现完整 UI 框架与 mock 数据层

- 安装 zustand + react-router-dom 依赖
- 修复 Vite 代理配置，添加 /auth 路由转发
- 实现 API 封装层（client.ts 统一 fetch、auth.ts 真实调用）
- 实现 mock 数据层（mock.ts 模拟 WebSocket 管线进度）
- 实现 4 个 zustand stores（auth/project/task/generation）
- 实现 5 个组件（StyleSelector/PromptEditor/GenerateForm/ProgressBar/AssetPreview）
- 实现 5 个页面（Login/Register/Project/Generate/Result）
- 实现路由含 ProtectedRoute 守卫
- 添加暗色主题全局样式
- 更新前端文档与 API 文档

frontend/src/api/client.ts

@@ -0,0 +1,70 @@
+import type { ApiResponse } from './types'
+
+const TOKEN_KEY = 'gen2d_token'
+
+export function getToken(): string | null {
+  return localStorage.getItem(TOKEN_KEY)
+}
+
+export function setToken(token: string): void {
+  localStorage.setItem(TOKEN_KEY, token)
+}
+
+export function clearToken(): void {
+  localStorage.removeItem(TOKEN_KEY)
+}
+
+export class ApiError extends Error {
+  code: number
+  constructor(code: number, message: string) {
+    super(message)
+    this.code = code
+  }
+}
+
+async function request<T>(url: string, options: RequestInit = {}): Promise<T> {
+  const token = getToken()
+  const headers: Record<string, string> = {
+    'Content-Type': 'application/json',
+    ...(options.headers as Record<string, string>),
+  }
+  if (token) {
+    headers['Authorization'] = `Bearer ${token}`
+  }
+
+  const res = await fetch(url, {
+    ...options,
+    headers,
+    credentials: 'include',
+  })
+
+  const json: ApiResponse<T> = await res.json()
+
+  if (json.code !== 0) {
+    if (json.code === 401) {
+      clearToken()
+      window.location.href = '/login'
+    }
+    throw new ApiError(json.code, json.message)
+  }
+
+  return json.data
+}
+
+export function get<T>(url: string): Promise<T> {
+  return request<T>(url)
+}
+
+export function post<T>(url: string, body?: unknown): Promise<T> {
+  return request<T>(url, {
+    method: 'POST',
+    body: body ? JSON.stringify(body) : undefined,
+  })
+}
+
+export function put<T>(url: string, body?: unknown): Promise<T> {
+  return request<T>(url, {
+    method: 'PUT',
+    body: body ? JSON.stringify(body) : undefined,
+  })
+}