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/Source.cs

@@ -48,9 +48,9 @@ public class Source : SceneElement
 
     public override bool IsWebSource => Type == SourceType.WebSource;
 
-    /// <summary>The permanent scene backdrop (live desktop/game capture). Never removable/reorderable.</summary>
-    private bool _isBackdrop;
-    public bool IsBackdrop { get => _isBackdrop; set => Set(ref _isBackdrop, value); }
+    /// <summary>The permanent scene background (live desktop/game capture). Never removable/reorderable.</summary>
+    private bool _isBackground;
+    public override bool IsBackground { get => _isBackground; set => Set(ref _isBackground, value); }
 
     private bool _showDesktop;
     public bool ShowDesktop { get => _showDesktop; set => Set(ref _showDesktop, value); }
@@ -67,7 +67,7 @@ public class Source : SceneElement
 
     /// <summary>
     /// Identifies what this live-capture source points at: "monitor:&lt;index&gt;" or
-    /// "window:&lt;hwnd&gt;". Persisted so a re-designated backdrop survives a reload.
+    /// "window:&lt;hwnd&gt;". Persisted so a re-designated background survives a reload.
     /// </summary>
     private string? _captureKey;
     public string? CaptureKey { get => _captureKey; set => Set(ref _captureKey, value); }