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

Services/ChatBoxRenderer.cs

@@ -24,6 +24,7 @@ public sealed class ChatBoxRenderer
         int width,
         int height,
         double fontSize,
+        string fontFamily,
         string fontColorHex,
         string backgroundColorHex,
         int maxMessages,
@@ -79,7 +80,7 @@ public sealed class ChatBoxRenderer
                     text,
                     InvariantCulture,
                     FlowDirection.LeftToRight,
-                    new Typeface(new FontFamily("Segoe UI"), FontStyles.Normal, FontWeights.Normal, FontStretches.Normal),
+                    new Typeface(new FontFamily(fontFamily), FontStyles.Normal, FontWeights.Normal, FontStretches.Normal),
                     fontSize,
                     brush,
                     dpi);