fix: guard against null actionManager during PlateView construction

Plate setter is called in the constructor before actionManager is
initialized, causing a NullReferenceException on startup.

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

OpenNest/Controls/PlateView.cs

@@ -202,7 +202,7 @@ namespace OpenNest.Controls
             foreach (var part in plate.Parts)
                 parts.Add(LayoutPart.Create(part, this));
 
-            actionManager.OnPlateChanged();
+            actionManager?.OnPlateChanged();
         }
 
         public string Status