fix: restore auto-navigation on plate add in PlateManager

OnPlateAdded now navigates to the new plate when suppressNavigation is
false, matching the original EditNestForm behavior. Fixed CanRemoveCurrent
test to account for this auto-navigation.

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

OpenNest.Core/PlateManager.cs

@@ -199,6 +199,12 @@ namespace OpenNest
         private void OnPlateAdded(object sender, ItemAddedEventArgs<Plate> e)
         {
             PlateListChanged?.Invoke(this, EventArgs.Empty);
+
+            if (!suppressNavigation)
+            {
+                CurrentIndex = Count - 1;
+                FireCurrentPlateChanged();
+            }
         }
 
         private void OnPlateRemoved(object sender, ItemRemovedEventArgs<Plate> e)