fix: handle null city for phone-registered users in dashboard

user.city is null for users registered via OTP — null assertion caused
crash showing grey DashboardLayout background.

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

lib/ui/views/dashboard_view.dart

@@ -117,7 +117,7 @@ class _DashboardViewState extends State<DashboardView> {
       );
     }
 
-    final _coords = citiesProvider.getCoordsOfCity(user!.city!);
+    final _coords = citiesProvider.getCoordsOfCity(user!.city ?? '');
 
     _createService() async {
       print('debug ${selectedProfessional?.user.id ?? 'user.id'}');