Add feedback service, reminder diagnostics and smooth week transitions

mobile/lib/services/notification_service.dart

@@ -22,4 +22,13 @@ abstract class NotificationService {
   );
   Future<void> requestPermissions();
   Future<void> cancelOwned();
+  Future<String> sendTestReminder({required bool scheduled}) async =>
+      '当前环境不支持通知测试';
+  Future<Map<String, Object?>> diagnose() async => {};
+  Future<void> openSettings({required bool background}) async {}
+}
+
+class ReminderSetupException implements Exception {
+  const ReminderSetupException(this.code, this.message);
+  final String code, message;
 }