Refactor card movement logic in BoardViewModel to improve error handling and success validation

- Updated the handling of card movement results to differentiate between successful and unsuccessful API responses.
- Implemented rollback of board state and error messaging for failed moves, ensuring a more robust user experience.
- Enhanced the notification system to surface relevant error messages when card movements fail.

lib/view_models/board_view_model.dart

@@ -104,10 +104,24 @@ class BoardViewModel with ChangeNotifier {
       newOrder: newOrder,
     );
 
-    if (result is Error<MoveCardResponse>) {
-      _board = _board!.copyWith(columns: previousColumns);
-      notifyListeners();
-    }
+    switch (result) {
+      case Ok<MoveCardResponse>():
+        // HTTP 200 alone isn't success: the API can return
+        // {"success": false, "message": ...} (e.g. column over-limit,
+        // permission denied). Only keep the optimistic move when the body
+        // confirms success; otherwise roll back and surface the message.
+        if (result.value.success == true) {
           return result;
         }
+        _board = _board!.copyWith(columns: previousColumns);
+        notifyListeners();
+        return Result.error(
+          Exception(result.value.message ?? 'Failed to move card'),
+        );
+      case Error<MoveCardResponse>():
+        _board = _board!.copyWith(columns: previousColumns);
+        notifyListeners();
+        return result;
+    }
+  }
 }