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

Models/SceneCatalog.cs

@@ -8,7 +8,7 @@ namespace ytLive.Models;
 /// <summary>
 /// The app's five canonical scenes. Scenes are worked on by name; you can delete
 /// them ("work with less"), but the app only ever adds back one of these five —
-/// anything beyond them is OBS territory. The live desktop/game backdrop exists
+/// anything beyond them is OBS territory. The live desktop/game background exists
 /// only in the Live scene.
 /// </summary>
 public static class SceneCatalog
@@ -47,6 +47,6 @@ public static class SceneCatalog
 
     public static bool IsChat(string? name) => Is(name, Chat);
 
-    /// <summary>Only the Live scene carries the live desktop/game backdrop.</summary>
-    public static bool HasBackdrop(string? name) => Is(name, Live);
+    /// <summary>Only the Live scene carries the live desktop/game background.</summary>
+    public static bool HasBackground(string? name) => Is(name, Live);
 }