fix: 修复 Flutter Web 生产环境 API 地址未生效的问题

- NetworkConfig.baseUrl 改为使用 ApiEndpoints.baseUrl
- 确保构建时 --dart-define=ENV=prod 能正确切换环境

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flutter_monisuo/lib/core/network/dio_client.dart

@@ -1,11 +1,12 @@
 import 'package:dio/dio.dart';
+import '../constants/api_endpoints.dart';
 import '../storage/local_storage.dart';
 import 'api_exception.dart';
 import 'api_response.dart';
 
 /// 网络配置常量
 class NetworkConfig {
-  static const String baseUrl = 'http://localhost:5010';
+  static const String baseUrl = ApiEndpoints.baseUrl;
   static const Duration connectTimeout = Duration(seconds: 30);
   static const Duration receiveTimeout = Duration(seconds: 30);
 }