fix(mobile-app): 修复 API 请求路径缺少 /api/v1 前缀问题

问题原因:
- 原 ApiClient baseUrl 为 'https://rwaapi.szaiai.com'
- 请求路径为 '/user/auto-create'
- 实际请求: https://rwaapi.szaiai.com/user/auto-create (错误)
- 正确路径: https://rwaapi.szaiai.com/api/v1/user/auto-create

解决方案:
- 新增 AppConfig 配置管理类
- 支持多环境配置 (dev/staging/prod)
- apiBaseUrl 已包含 /api/v1 前缀
- 支持 --dart-define=ENV=dev 切换环境

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/mobile-app/lib/core/config/app_config.dart

@@ -0,0 +1,95 @@
+import 'package:flutter/foundation.dart';
+
+/// 应用环境
+enum AppEnvironment {
+  development,
+  staging,
+  production,
+}
+
+/// 应用配置
+///
+/// 根据环境变量或编译配置决定 API 地址等配置
+class AppConfig {
+  static AppConfig? _instance;
+
+  final AppEnvironment environment;
+  final String apiBaseUrl;
+  final bool enableLogging;
+
+  AppConfig._({
+    required this.environment,
+    required this.apiBaseUrl,
+    required this.enableLogging,
+  });
+
+  /// 初始化配置
+  ///
+  /// 可通过以下方式指定环境:
+  /// 1. flutter run --dart-define=ENV=dev
+  /// 2. 编译时: flutter build apk --dart-define=ENV=prod
+  static void initialize() {
+    const envString = String.fromEnvironment('ENV', defaultValue: 'prod');
+
+    final environment = _parseEnvironment(envString);
+
+    _instance = AppConfig._(
+      environment: environment,
+      apiBaseUrl: _getApiBaseUrl(environment),
+      enableLogging: environment != AppEnvironment.production,
+    );
+
+    if (kDebugMode) {
+      debugPrint('🔧 [AppConfig] Environment: ${_instance!.environment}');
+      debugPrint('🔧 [AppConfig] API Base URL: ${_instance!.apiBaseUrl}');
+    }
+  }
+
+  /// 获取配置实例
+  static AppConfig get instance {
+    if (_instance == null) {
+      initialize();
+    }
+    return _instance!;
+  }
+
+  /// 解析环境字符串
+  static AppEnvironment _parseEnvironment(String env) {
+    switch (env.toLowerCase()) {
+      case 'dev':
+      case 'development':
+        return AppEnvironment.development;
+      case 'staging':
+      case 'stage':
+        return AppEnvironment.staging;
+      case 'prod':
+      case 'production':
+      default:
+        return AppEnvironment.production;
+    }
+  }
+
+  /// 获取 API 基础地址
+  static String _getApiBaseUrl(AppEnvironment env) {
+    switch (env) {
+      case AppEnvironment.development:
+        // Android 模拟器访问本机: 10.0.2.2
+        // iOS 模拟器访问本机: localhost
+        // 真机测试: 使用局域网 IP
+        return const String.fromEnvironment(
+          'API_URL',
+          defaultValue: 'http://10.0.2.2:3000/api/v1',
+        );
+      case AppEnvironment.staging:
+        return 'https://rwaapi-dev.szaiai.com/api/v1';
+      case AppEnvironment.production:
+        return 'https://rwaapi.szaiai.com/api/v1';
+    }
+  }
+
+  /// 是否为开发环境
+  bool get isDevelopment => environment == AppEnvironment.development;
+
+  /// 是否为生产环境
+  bool get isProduction => environment == AppEnvironment.production;
+}

frontend/mobile-app/lib/core/network/api_client.dart

@@ -3,6 +3,7 @@ import 'package:flutter/foundation.dart';
 import '../storage/secure_storage.dart';
 import '../storage/storage_keys.dart';
 import '../errors/exceptions.dart';
+import '../config/app_config.dart';
 
 /// API 客户端
 ///
@@ -30,10 +31,8 @@ class ApiClient {
     _setupInterceptors();
   }
 
-  // 生产环境 API 地址
+  // 从配置获取 API 地址
-  static const String _defaultBaseUrl = 'https://rwaapi.szaiai.com';
+  static String get _defaultBaseUrl => AppConfig.instance.apiBaseUrl;
-  // 开发环境 (Android 模拟器访问本机): 'http://10.0.2.2:3000'
-  // 开发环境 (iOS 模拟器访问本机): 'http://localhost:3000'
 
   /// 设置拦截器
   void _setupInterceptors() {