Report delivered device count from the test-notification endpoint

POST /api/notifications/test previously returned a blanket 200 even when there
were no registered devices, so a test that reached nobody looked like a success.
push::send now returns the number of devices the relay confirmed delivery to, and
the endpoint returns it as {"delivered": N}. Settings shows "sent to N device(s)"
on success and an explicit "No devices are registered..." warning when N is 0,
which is the case that previously masqueraded as success.

Backend, API and widget tests updated; clippy and dart analyze clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/settings/settings.dart

@@ -143,12 +143,22 @@ class _SettingsState extends State<Settings> {
   Future<void> _sendTestNotification() async {
     setState(() => _testBusy = true);
     try {
-      await BackendAPI.instance.sendTestNotification();
+      final delivered = await BackendAPI.instance.sendTestNotification();
       if (!mounted) return;
-      UISnackbars.showSuccess(
-        context,
-        'Test notification sent. It should arrive shortly.',
-      );
+      if (delivered == 0) {
+        // The request succeeded but no device received it — usually the backend
+        // lost this device's token (e.g. after a restart); re-toggle to re-register.
+        UISnackbars.showError(
+          context,
+          'No devices are registered to receive push notifications.',
+        );
+      } else {
+        final devices = delivered == 1 ? 'device' : 'devices';
+        UISnackbars.showSuccess(
+          context,
+          'Test notification sent to $delivered $devices.',
+        );
+      }
     } catch (e) {
       debugPrint('Failed to send test notification: $e');
       if (!mounted) return;

frontend/lib/utils/api/oott_api_notifications.dart

@@ -15,9 +15,11 @@ extension NotificationApi on BackendAPI {
   }
 
   /// Asks the backend to deliver a one-off test push to every registered device,
-  /// used from Settings to verify end-to-end push delivery.
-  Future<void> sendTestNotification() async {
-    await _dio.post('/notifications/test');
+  /// used from Settings to verify end-to-end push delivery. Returns the number of
+  /// devices the relay confirmed delivery to (0 when none are registered).
+  Future<int> sendTestNotification() async {
+    final response = await _dio.post('/notifications/test');
+    return (response.data as Map<String, dynamic>)['delivered'] as int;
   }
 
   Future<({List<Notification> items, int totalCount})> listNotifications(

frontend/test/api/notifications_api_test.dart

@@ -36,13 +36,13 @@ void main() {
     await BackendAPI.instance.markAllNotificationsAsRead();
   });
 
-  test('sendTestNotification POSTs to /notifications/test', () async {
+  test('sendTestNotification POSTs and returns the delivered count', () async {
     adapter.onPost(
       '/notifications/test',
-      (server) => server.reply(200, null),
+      (server) => server.reply(200, {'delivered': 2}),
     );
 
-    await BackendAPI.instance.sendTestNotification();
+    expect(await BackendAPI.instance.sendTestNotification(), 2);
   });
 
   test(

frontend/test/widget/settings_push_test.dart

@@ -173,11 +173,13 @@ void main() {
     expect(find.text('Send test notification'), findsNothing);
   });
 
-  testWidgets('sends a test notification when push is on', (tester) async {
+  testWidgets('reports the device count when a test notification is sent', (
+    tester,
+  ) async {
     stubNotificationMethod('push');
     adapter.onPost(
       '/notifications/test',
-      (server) => server.reply(200, null),
+      (server) => server.reply(200, {'delivered': 2}),
     );
     await PrefUtil.setValue('push_enabled', true);
     await pumpScreen(tester, Settings(pushService: _FakePushService()));
@@ -186,7 +188,26 @@ void main() {
     await tester.tap(find.text('Send test notification'));
     await pumpUntilFound(
       tester,
-      find.text('Test notification sent. It should arrive shortly.'),
+      find.text('Test notification sent to 2 devices.'),
+    );
+  });
+
+  testWidgets('warns when a test notification reaches no devices', (
+    tester,
+  ) async {
+    stubNotificationMethod('push');
+    adapter.onPost(
+      '/notifications/test',
+      (server) => server.reply(200, {'delivered': 0}),
+    );
+    await PrefUtil.setValue('push_enabled', true);
+    await pumpScreen(tester, Settings(pushService: _FakePushService()));
+    await pumpUntilFound(tester, find.text('Send test notification'));
+
+    await tester.tap(find.text('Send test notification'));
+    await pumpUntilFound(
+      tester,
+      find.text('No devices are registered to receive push notifications.'),
     );
   });
 }