fix: 添加安装路由，修复环境变量校验和路由守卫

- 注册/install路由，安装向导页面可访问
- env.ts: VITE_SERVER_API_URL默认空字符串(相对路径)
- 路由守卫: API失败时默认跳转安装页面

frontend/src/router/routes.ts

@@ -1,6 +1,15 @@
 import type { RouteRecordRaw } from 'vue-router'
 
 const routes: RouteRecordRaw[] = [
+  {
+    path: '/install',
+    name: 'Install',
+    component: () => import('@/pages/install/index.vue'),
+    meta: {
+      title: '安装 - 管理平台',
+      layout: false,
+    },
+  },
   {
     path: '/',
     redirect: '/login',