feat: show selected part bounding box in status bar

Add SelectionChanged event to PlateView and display the selected part's
location and size in a new status bar label. Shows combined bounding box
when multiple parts are selected.

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

OpenNest/Actions/ActionSelect.cs

@@ -171,6 +171,7 @@ namespace OpenNest.Actions
                 part.IsSelected = true;
             }
 
+            plateView.NotifySelectionChanged();
             plateView.Invalidate();
             return true;
         }
@@ -187,6 +188,8 @@ namespace OpenNest.Actions
                 plateView.SelectedParts.Add(part);
                 part.IsSelected = true;
             }
+
+            plateView.NotifySelectionChanged();
         }
 
         private void UpdateBrushAndPen()