fix(app): Flutter Web 开发模式默认连接 localhost:3000 API

- app.dart: kDebugMode 下使用 AppConfig.dev (localhost:3000)
- dev.mjs: flutter run 传入 --dart-define API_BASE_URL/SSE_BASE_URL

app/lib/app.dart

@@ -10,7 +10,7 @@
 // └─ BlocProvider<AuthBloc>
 //     └─ MaterialApp.router
 
-import 'package:flutter/foundation.dart' show kIsWeb;
+import 'package:flutter/foundation.dart' show kIsWeb, kDebugMode;
 import 'package:flutter/material.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:go_router/go_router.dart';
@@ -37,7 +37,8 @@ class NuanjiApp extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     // 创建全局依赖（App 生命周期内单例）
-    final config = AppConfig.fromEnvironment();
+    // 开发模式默认使用 localhost，可通过 --dart-define 覆盖
+    final config = kDebugMode ? AppConfig.dev : AppConfig.fromEnvironment();
     final apiClient = ApiClient(baseUrl: config.apiBaseUrl);
     final tokenStore = createSecureTokenStore();
     final authRepository = AuthRepository(apiClient: apiClient, tokenStore: tokenStore);