chore(notifications): log push token registration outcome for debugging

Add debugPrint entries before and after POST /universal-notifications/push
so we can trace whether a given token was accepted by the backend
during debugging of push delivery issues.

packages/sf_shared/lib/src/data/datasource/notifications_remote_datasource_impl.dart

@@ -1,5 +1,6 @@
 import 'package:dio/dio.dart';
 import 'package:firebase_messaging/firebase_messaging.dart';
+import 'package:flutter/foundation.dart';
 import 'package:sf_infrastructure/sf_infrastructure.dart';
 
 import 'notifications_remote_datasource.dart';
@@ -19,11 +20,13 @@ class NotificationsRemoteDatasourceImpl
 
   @override
   Future<void> registerPushToken(String token) async {
+    debugPrint('[FCM] registering token: $token');
     try {
       await _repository.post<Map<String, dynamic>>(
         '/universal-notifications/push',
         body: <String, dynamic>{'token': token},
       );
+      debugPrint('[FCM] token registered successfully');
     } on DioException catch (error) {
       final apiMsg = error.response?.data;
       final msg = apiMsg is String