tested version with most of the final functionality. Still a few known issues, like dragging to a row that extends to multiple lines

RowCardContainer.cs

@@ -14,7 +14,7 @@ public partial class RowCardContainer : HFlowContainer
 	}
 	public override bool _CanDropData(Vector2 atPosition, Variant data)
     {
-        return data.As<card>() is not null;
+        return data.Obj is card;
     }
     public override void _DropData(Vector2 atPosition, Variant data)
     {