feat(ui): Show Desktop toggle, avatar login, chat preview, backdrop seeding, border animations

- Source.ShowDesktop bool persisted in DB (schema migration + LayoutStore)
- ClearBackdropCaptureAsync clears CaptureKey + VideoImageSource for static fallback
- Top bar: red/green/pulsing status light, avatar via BitmapImage, Log In button
- ShowStartStream now requires IsConnected
- RunMockChatPreviewAsync: simple async loop, ChatPreviewEnabled property on Source
- EnsureDefaultBackdrop called on every scene switch, seeds all 5 scenes
- SeedEndingBackdrop replaces existing, SeedChatBackdrop added
- SceneCatalog: Chat scene display name corrected (layer = 'YouTube Chat')
- Left panel spacing consistency, context menu reordered
- Border animation rendering via code-behind Storyboard approach
- 3 new backdrop assets (ending-backdrop.jpg, chat-backdrop.jpg, live-backdrop.jpg)
- Build 0 warnings, 178 tests passing

ytLive.Tests/BackdropTests.cs

@@ -91,6 +91,13 @@ public class BackdropTests
         Assert.Same(bitmap, source.DisplaySource);
     }
 
+    [Fact]
+    public void Source_DisplaySource_FallsBackToImage_WhenLiveCaptureWithNoVideoImage()
+    {
+        var source = new Source { Type = SourceType.DisplayCapture };
+        Assert.Null(source.DisplaySource);
+    }
+
     [Fact]
     public void Source_DisplaySource_IsImageSource_WhenNotLiveCapture()
     {