feat: Add WebSocket URL to env.example and refine notification handling logic

env.example

···
-API_URL=https://grain.social
+API_URL=https://grain.social
+WS_URL=wss://notifications.grainsocial.network/ws

lib/providers/notifications_provider.dart

···
             final notifications = await apiService.getNotifications();
             state = notifications;
           } else {
-            // You may need to adjust this if your WS sends a list or other format
             final notification = Notification.fromJson(data);
             state = [...state, notification];
           }