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

ytLive.Tests/RoundClipInteractionTests.cs

@@ -48,7 +48,7 @@ public sealed class RoundClipInteractionTests
             window.Show();
             window.UpdateLayout();
 
-            var scene = vm.ActiveScene!;
+            var scene = vm.StagedScene!;
             var webcam = new WebcamSceneConfig
             {
                 WebcamId = "test-camera",