Control Surface UX: pill toggle, background rename, per-scene properties

- Sliding pill toggle (green ON / red OFF) replaces checkbox
- Geometry mismatch error for custom backgrounds
- Backdrop -> Background: C# rename, SQL column rename with migration
- Source.IsBackground: new -> override (fixes WPF binding)
- Legacy 'Backdrop' display name migrated to 'Background'
- Per-scene UseDefaultBackground + CustomBackgroundPath
- Asset files renamed backdrop -> background
- StagedScene/LiveScene split, thumbnail strip, edit gating
- TransitionService (Cut/Fade/Move)
- Context menu: Capture Desktop, Refresh Desktop
- Resolution badge removed, ON-AIR status light added

Services/YouTubeStreamService.cs

@@ -111,7 +111,7 @@ public class YouTubeStreamService
 
         var stream = new
         {
-            snippet = new { title = "ytLlive Reusable Stream" },
+            snippet = new { title = "LlamaCasty Reusable Stream" },
             cdn = new
             {
                 ingestionType = "rtmp",