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/ScreenCaptureManager.cs

@@ -158,7 +158,7 @@ public sealed class ScreenCaptureManager : IDisposable
 
     /// <summary>The most recent frame for a capture key, or null before the first
     /// frame arrives (or if the key has no session). The live compositor reads
-    /// the backdrop from here.</summary>
+    /// the background from here.</summary>
     public VideoFrame? GetLatestFrame(string key)
     {
         lock (_gate)