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/SocialValidator.cs

@@ -46,7 +46,7 @@ public sealed class HttpSocialValidator : ISocialValidator
     {
         _client = client ?? new HttpClient();
         _client.DefaultRequestHeaders.UserAgent.ParseAdd(
-            "Mozilla/5.0 (compatible; ytLlive social validator)");
+            "Mozilla/5.0 (compatible; LlamaCasty social validator)");
         _log = log;
     }