fix: marshal timer callbacks to UI thread to prevent GDI+ threading exception

System.Timers.Timer fires on thread pool threads, causing GraphicsPath
objects to be accessed concurrently by hover detection and OnPaint,
triggering "Object is currently in use elsewhere" in DrawParts.

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

OpenNest/Controls/PlateView.cs

@@ -621,15 +621,21 @@ namespace OpenNest.Controls
 
         private void redrawTimer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
         {
-            Invalidate();
+            if (IsDisposed || !IsHandleCreated) return;
+            BeginInvoke(new System.Action(Invalidate));
         }
 
         private void hoverTimer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
+        {
+            if (IsDisposed || !IsHandleCreated) return;
+            BeginInvoke(new System.Action(HoverCheck));
+        }
+
+        private void HoverCheck()
         {
             var graphPt = PointControlToGraph(hoverPoint);
             LayoutPart hitPart = null;
-            try
+
-            {
             for (var i = parts.Count - 1; i >= 0; --i)
             {
                 if (parts[i].Path.GetBounds().Contains(graphPt) &&
@@ -639,12 +645,6 @@ namespace OpenNest.Controls
                     break;
                 }
             }
-            }
-            catch (InvalidOperationException)
-            {
-                // GraphicsPath in use by paint thread — skip this hover tick
-                return;
-            }
 
             hoveredPart = hitPart;
             showTooltip = hitPart != null;