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/SceneElement.cs

@@ -61,6 +61,7 @@ public abstract class SceneElement : INotifyPropertyChanged
 
     /// <summary>True only for Countdown-type sources. Gates timer properties panel.</summary>
     public virtual bool IsCountdown => false;
+    public virtual bool IsBackground { get; set; }
 
     private bool _isVisible = true;
     public bool IsVisible { get => _isVisible; set => Set(ref _isVisible, value); }