feat(client): 登录模块对接后端 API

- auth_state: 增加 refreshToken 字段 + flutter_secure_storage 持久化
- api_client: 增加 401 自动 refresh + retry 拦截器
- auth_repository: 封装 POST /api/v1/auth/login 调用，错误信息本地化
- login_screen: 增加门店编号字段，调真实 API，行内错误展示
- main: 启动时 restore 持久化登录态，避免每次重启都要重新登录
- 修复 app_shell: hotelName → hotelNo
- 修复 widget_test: 移除失效的占位测试

测试账号：门店编号 H001 / 用户名 admin / 密码 password123

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

client/lib/repositories/auth_repository.dart

@@ -0,0 +1,56 @@
+import 'package:dio/dio.dart';
+import '../core/api/api_client.dart';
+import '../core/auth/auth_state.dart';
+
+class AuthException implements Exception {
+  final String message;
+  const AuthException(this.message);
+  @override
+  String toString() => message;
+}
+
+class AuthRepository {
+  /// POST /api/v1/auth/login
+  /// Request:  { hotel_code, username, password }
+  /// Response: { data: { access_token, refresh_token, expires_in, user: { id, username, real_name, role } } }
+  static Future<AuthUser> login({
+    required String hotelCode,
+    required String username,
+    required String password,
+  }) async {
+    try {
+      final resp = await PublicApiClient.post('/auth/login', data: {
+        'hotel_code': hotelCode,
+        'username': username,
+        'password': password,
+      });
+
+      final data = resp.data['data'] as Map<String, dynamic>;
+      final user = data['user'] as Map<String, dynamic>;
+
+      return AuthUser(
+        accessToken: data['access_token'] as String,
+        refreshToken: data['refresh_token'] as String,
+        username: user['username'] as String,
+        realName: user['real_name'] as String? ?? username,
+        hotelNo: hotelCode,
+        hotelId: (user['id'] as num).toInt(),
+      );
+    } on DioException catch (e) {
+      final msg = e.response?.data?['error'] as String?;
+      throw AuthException(msg ?? _networkError(e));
+    }
+  }
+
+  static String _networkError(DioException e) {
+    switch (e.type) {
+      case DioExceptionType.connectionTimeout:
+      case DioExceptionType.receiveTimeout:
+        return '连接超时，请检查网络';
+      case DioExceptionType.connectionError:
+        return '无法连接到服务器（localhost:8080），请先启动后端';
+      default:
+        return '网络错误：${e.message}';
+    }
+  }
+}