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/stores/auth.ts

@@ -0,0 +1,30 @@
+import { defineStore } from 'pinia'
+
+interface AuthState {
+  token: string | null
+  username: string | null
+}
+
+export const useAuthStore = defineStore('auth', {
+  state: (): AuthState => ({
+    token: localStorage.getItem('token'),
+    username: localStorage.getItem('username'),
+  }),
+  getters: {
+    isAuthenticated: (state): boolean => !!state.token,
+  },
+  actions: {
+    setToken(token: string, username: string): void {
+      this.token = token
+      this.username = username
+      localStorage.setItem('token', token)
+      localStorage.setItem('username', username)
+    },
+    logout(): void {
+      this.token = null
+      this.username = null
+      localStorage.removeItem('token')
+      localStorage.removeItem('username')
+    },
+  },
+})