feat: more easter eggs + version checking

lib/viewmodels/remote_sync_view_model.dart

@@ -29,6 +29,12 @@ class RemoteSyncViewModel extends ChangeNotifier {
   /// The server rejected this device's token; it needs to be linked again.
   bool get wasRevoked => _wasRevoked;
 
+  String? _updateRequired;
+
+  /// The backoffice refused this app version, with its message: the app has
+  /// to be updated before it syncs again.
+  String? get updateRequired => _updateRequired;
+
   DateTime? _lastSyncAt;
   DateTime? get lastSyncAt => _lastSyncAt;
 
@@ -130,12 +136,15 @@ class RemoteSyncViewModel extends ChangeNotifier {
         _lastSyncAt = DateTime.now();
         _lastError = null;
         _isOffline = false;
+        _updateRequired = null;
       case RemoteSyncEventType.syncFailed:
         _lastError = event.message;
         _isOffline = event.offline;
       case RemoteSyncEventType.unauthorized:
         _isConfigured = false;
         _wasRevoked = true;
+      case RemoteSyncEventType.updateRequired:
+        _updateRequired = event.message ?? '';
       case RemoteSyncEventType.tabClosedRemotely:
       case RemoteSyncEventType.conflict:
         break;