feat(frontend): S0 Vue3 脚手架 + auth + Layout shell（4入口，模拟/实盘灰显）

- Vite+TS+ElementPlus+ECharts+Pinia+Router+Axios
- auth store + JWT axios 拦截器 + 路由守卫
- 登录页 + Layout（侧栏 4 入口，回测/投研 active，模拟/实盘 disabled）
- 占位页（backtest/factor，S1/S2 填充）
- vitest auth store 3 tests passed + build 通过(dist 生成)

frontend/src/api/client.ts

@@ -0,0 +1,28 @@
+import axios, { AxiosError } from 'axios'
+import { useAuthStore } from '@/stores/auth'
+import { router } from '@/router'
+
+export const apiClient = axios.create({
+  baseURL: '/api/v1',
+  timeout: 60000,
+})
+
+apiClient.interceptors.request.use((config) => {
+  const auth = useAuthStore()
+  if (auth.token) {
+    config.headers.Authorization = `Bearer ${auth.token}`
+  }
+  return config
+})
+
+apiClient.interceptors.response.use(
+  (response) => response,
+  (error: AxiosError) => {
+    if (error.response?.status === 401) {
+      const auth = useAuthStore()
+      auth.logout()
+      router.push('/login')
+    }
+    return Promise.reject(error)
+  },
+)