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

Helpers/HiddenBoolToVisibilityConverter.cs

@@ -7,7 +7,7 @@ namespace ytLive.Helpers;
 /// <summary>
 /// Maps bool → Visibility, collapsing nothing: true becomes <c>Hidden</c> (keeps
 /// its layout slot) rather than <c>Collapsed</c> (releases it). Used where a row
-/// must keep its column grid aligned — e.g. the Backdrop source's missing
+/// must keep its column grid aligned — e.g. the Background source's missing
 /// trashcan must not shift the edit/eye icons one slot to the right.
 /// </summary>
 public class HiddenBoolToVisibilityConverter : IValueConverter