feat: 实现基础设施层

axios 请求封装及七个业务模块 API，Pinia 状态管理（auth/system/models/tools），Mock 适配器与数据，以及流式对话、轮询、倒计时组合式函数。

frontend/src/stores/auth.ts

@@ -0,0 +1,45 @@
+import { defineStore } from 'pinia'
+import { ref, computed } from 'vue'
+import { login as loginApi } from '@/api/modules/system'
+import { SESSION_TIMEOUT } from '@/constants'
+
+/**
+ * 认证 store
+ * 沿用原项目 localStorage 的登录时间戳 + 5 分钟会话超时机制
+ */
+export const useAuthStore = defineStore('auth', () => {
+  const username = ref<string>(localStorage.getItem('username') || '')
+  const loginTime = ref<number>(parseInt(localStorage.getItem('loginTime') || '0', 10) || 0)
+
+  const isLoggedIn = computed(() => {
+    if (!loginTime.value) return false
+    return Date.now() - loginTime.value < SESSION_TIMEOUT
+  })
+
+  /** 登录 */
+  async function login(user: string, password: string) {
+    await loginApi(user, password)
+    username.value = user
+    loginTime.value = Date.now()
+    localStorage.setItem('username', user)
+    localStorage.setItem('loginTime', String(loginTime.value))
+  }
+
+  /** 续期会话（活跃时刷新） */
+  function refresh() {
+    if (isLoggedIn.value) {
+      loginTime.value = Date.now()
+      localStorage.setItem('loginTime', String(loginTime.value))
+    }
+  }
+
+  /** 退出 */
+  function logout() {
+    username.value = ''
+    loginTime.value = 0
+    localStorage.removeItem('username')
+    localStorage.removeItem('loginTime')
+  }
+
+  return { username, loginTime, isLoggedIn, login, refresh, logout }
+})