refactor: extract SceneThumbnailStrip out of MainWindow.xaml — Phase 2, Commit 16

SceneThumbnailStrip.xaml(.cs) takes the Row 0 scene-picker strip (ItemsControl
over Scenes with staged/live border highlight); Thumbnail_MouseLeftButtonDown
moved into the control, transitioning via DataContext.TransitionToScene.

Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md Phase 2 tracker updated in the same commit.

MainWindow.xaml.cs

@@ -164,13 +164,6 @@ public partial class MainWindow : Window
         }
     }
 
-    // ─── Preview image selection / move / resize ───
-    private void Thumbnail_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
-    {
-        if (sender is Border { DataContext: Scene scene })
-            _viewModel.TransitionToScene(scene);
-    }
-
 
     private void Window_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
     {