Fix: Elements scrollbar click no longer deselects the selected element

The scrollbar lives inside the ScrollViewer, not the StackPanel
(ElementsPanel). Window_PreviewMouseLeftButtonDown was checking
IsDescendantOf against ElementsPanel, so scrollbar clicks fell through
and nulled SelectedElement. Now checks ElementsScrollViewer instead.

1a of 7-issue polish batch. 207 tests, 0 warnings.

MainWindow.xaml.cs

@@ -209,7 +209,7 @@ public partial class MainWindow : Window
         if (e.OriginalSource is not DependencyObject original) return;
         if (IsDescendantOf(original, PreviewGrid)) return;
         if (IsDescendantOf(original, SourceList)) return;
-        if (IsDescendantOf(original, ElementsPanel)) return;
+        if (IsDescendantOf(original, ElementsScrollViewer)) return;
         _viewModel.SelectedElement = null;
     }