fix: clear part selection highlight when leaving lead-in action

ActionLeadIn.DisconnectEvents() nulled selectedLayoutPart without first
setting IsSelected = false, leaving the part permanently rendered in the
selection color (transparent blue) after switching actions.

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

OpenNest/Actions/ActionLeadIn.cs

@@ -65,6 +65,9 @@ namespace OpenNest.Actions
             contextMenu?.Dispose();
             contextMenu = null;
 
+            if (selectedLayoutPart != null)
+                selectedLayoutPart.IsSelected = false;
+
             selectedLayoutPart = null;
             selectedPart = null;
             profile = null;