fix: re-enable delete plate button when changing plate selection

UpdateRemovePlateButton() was only called from PlateListChanged,
not CurrentPlateChanged, so the button stayed disabled after switching
away from the sentinel plate.

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

OpenNest/Forms/EditNestForm.cs

@@ -892,6 +892,7 @@ namespace OpenNest.Forms
             PlateView.Plate = PlateManager.CurrentPlate;
             PlateView.ZoomToFit();
             UpdatePlateHeader();
+            UpdateRemovePlateButton();
             PlateChanged?.Invoke(this, EventArgs.Empty);
         }