fix: prevent Delete key from corrupting quantity during ActionClone

ObservableList.Remove fired ItemRemoved even when the item wasn't in
the list, causing Plate to decrement Quantity.Nested for clone preview
parts that were never added — producing -1 counts. Delete in PlateView
now cancels ActionClone instead of trying to remove its preview parts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Core/Collections/ObservableList.cs

@@ -46,7 +46,8 @@ namespace OpenNest.Collections
         public bool Remove(T item)
         {
             var success = items.Remove(item);
-            ItemRemoved?.Invoke(this, new ItemRemovedEventArgs<T>(item, success));
+            if (success)
+                ItemRemoved?.Invoke(this, new ItemRemovedEventArgs<T>(item, success));
             return success;
         }
 

OpenNest/Controls/PlateView.cs

@@ -386,7 +386,11 @@ namespace OpenNest.Controls
             switch (e.KeyCode)
             {
                 case Keys.Delete:
-                    if (selectedCutOff != null)
+                    if (currentAction is ActionClone)
+                    {
+                        SetAction(typeof(ActionSelect));
+                    }
+                    else if (selectedCutOff != null)
                     {
                         Plate.CutOffs.Remove(selectedCutOff);
                         selectedCutOff = null;