fix: stop plate list changes from forcing tab switch

PlateListChanged handler was setting tabControl1.SelectedIndex = 0,
which forced the UI to the plates tab whenever a sentinel plate was
auto-created during part placement, disrupting the workflow.

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

OpenNest/Forms/EditNestForm.cs

@@ -899,7 +899,6 @@ namespace OpenNest.Forms
 
         private void PlateManager_PlateListChanged(object sender, EventArgs e)
         {
-            tabControl1.SelectedIndex = 0;
             UpdatePlateList();
             UpdatePlateHeader();
             UpdateRemovePlateButton();