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/utils/env.ts

@@ -9,10 +9,8 @@ import { z } from 'zod'
  */
 
 const EnvSchema = z.object({
-  // Add your environment variables here, for example:
-  // VITE_API_BASE_URL: z.string().url(),
-  VITE_SERVER_API_URL: z.url(),
-  VITE_SERVER_API_PREFIX: z.string(),
+  VITE_SERVER_API_URL: z.string().default(''),
+  VITE_SERVER_API_PREFIX: z.string().default('/api'),
   VITE_SERVER_API_TIMEOUT: z.coerce.number().default(5000),
 })