Scene strip: move top row over the preview column and scale it; remove CRT bezel art and on-air seed

- PreviewPane.xaml: drop the OBS-style border Rectangle/Ellipse over webcam elements
- SceneThumbnailStrip: Viewbox scales the row; height follows width so it fills the preview column; staged+live tile has no border
- MainWindow.xaml: strip moved to column 1 only, leaving the layers cell empty; Margin 8,0 matches the preview
- MainViewModel.Background: stop seeding the CRT scene tiles and the on-air image
- ytLive.csproj: per-build counter (BuildCounter.txt, gitignored) replaces the commit-count stamp

Build stamp now increments on every build.

Controls/SceneThumbnailStrip.xaml.cs

@@ -8,9 +8,12 @@ namespace ytLive.Controls;
 
 public partial class SceneThumbnailStrip : UserControl
 {
+    private const double DesignAspect = 154.0 / 1000.0;
+
     public SceneThumbnailStrip()
     {
         InitializeComponent();
+        SizeChanged += (_, _) => Height = ActualWidth * DesignAspect;
     }
 
     private void Thumbnail_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)