fix(mining-admin-web): always use /api proxy instead of direct API URL

Browser cannot access Docker internal URLs like http://mining-admin-service:3023.
Always use /api which is proxied by Next.js rewrites to the backend service.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mining-admin-web/src/lib/api/client.ts

@@ -1,7 +1,7 @@
 import axios from 'axios';
 
-// 生产环境使用 NEXT_PUBLIC_API_URL，开发环境使用 /api 代理
+// 始终使用 /api 代理，由 Next.js rewrites 转发到后端服务
-const baseURL = process.env.NEXT_PUBLIC_API_URL || '/api';
+const baseURL = '/api';
 
 export const apiClient = axios.create({
   baseURL,