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

SocialsDialog.xaml.cs

@@ -41,7 +41,7 @@ public partial class SocialsDialog : Window
         if (sender is not FrameworkElement { Tag: int index } element) return;
         var slot = element.DataContext as SocialSlotViewModel;
         var confirm = index == 0
-            ? "Remove your YouTube channel from the bar and sign out of ytLlive?"
+            ? "Remove your YouTube channel from the bar and sign out of LlamaCasty?"
             : slot is { IsFilled: true }
                 ? $"Remove {slot.Handle}?"
                 : "Remove this social?";