add notification, fix appbar,fix style subscription

lib/domain/usecase/get_notifications_usecase.dart

@@ -0,0 +1,12 @@
+import '../entities/client_notification.dart';
+import '../repositories/notification_repository.dart';
+
+class GetNotificationsUsecase {
+  final NotificationRepository repository;
+
+  GetNotificationsUsecase(this.repository);
+
+  Future<List<ClientNotification>> call() {
+    return repository.getNotifications();
+  }
+}