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.

MainWindow.xaml

@@ -78,8 +78,8 @@
                 <ColumnDefinition Width="3*"/>
             </Grid.ColumnDefinitions>
 
-            <!-- ═══ THUMBNAIL STRIP ═══ -->
-            <controls:SceneThumbnailStrip Grid.Row="0" Grid.ColumnSpan="2"/>
+            <!-- ═══ THUMBNAIL STRIP (over the preview column; left column stays empty) ═══ -->
+            <controls:SceneThumbnailStrip Grid.Row="0" Grid.Column="1" Margin="8,0"/>
 
             <!-- LEFT PANEL: one panel, two states.
                  Offline (edit mode): LAYERS + PROPERTIES