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;