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

ytLive.Tests/FramePumpTests.cs

@@ -59,10 +59,10 @@ public class FramePumpTests
         public void RaiseHealth(StreamHealth health) => HealthUpdated?.Invoke(this, health);
     }
 
-    private static Scene BackdropScene()
+    private static Scene BackgroundScene()
     {
         var scene = new Scene { Name = "Live" };
-        scene.Elements.Add(new Source { Type = SourceType.DisplayCapture, IsBackdrop = true, CaptureKey = "monitor:0" });
+        scene.Elements.Add(new Source { Type = SourceType.DisplayCapture, IsBackground = true, CaptureKey = "monitor:0" });
         return scene;
     }
 
@@ -72,7 +72,7 @@ public class FramePumpTests
         Func<(VideoFrame? Frame, SocialBarPosition Position)>? socialBar = null)
     {
         return new FramePump(
-            sceneProvider: scene ?? (() => BackdropScene()),
+            sceneProvider: scene ?? (() => BackgroundScene()),
             frameResolver: resolve ?? (_ => null),
             compositorOptions: () => new CompositorOptions
             {
@@ -166,7 +166,7 @@ public class FramePumpTests
         var red = SceneCompositorTests.Solid(64, 48, 255, 0, 0);
         var encoder = new FakeEncoder();
         using var pump = NewPump(encoder,
-            resolve: e => e is Source { IsBackdrop: true } ? red : null);
+            resolve: e => e is Source { IsBackground: true } ? red : null);
 
         await pump.StartAsync();
         Assert.True(pump.IsRunning);
@@ -177,7 +177,7 @@ public class FramePumpTests
         var frame = encoder.Frames[0];
         Assert.Equal(64, frame.Width);
         Assert.Equal(48, frame.Height);
-        AssertColor(frame, 0, 0, 255, 0, 0); // the backdrop really was composited in
+        AssertColor(frame, 0, 0, 255, 0, 0); // the background really was composited in
 
         await pump.StopAsync();
         Assert.False(pump.IsRunning);
@@ -236,7 +236,7 @@ public class FramePumpTests
         var encoder = new FakeEncoder();
         var position = SocialBarPosition.Top;
         using var pump = NewPump(encoder,
-            resolve: e => e is Source { IsBackdrop: true } ? red : null,
+            resolve: e => e is Source { IsBackground: true } ? red : null,
             socialBar: () => (bar, position));
 
         await pump.StartAsync();
@@ -246,7 +246,7 @@ public class FramePumpTests
 
         // Flip to Bottom: the pump re-reads the seam each frame, so a later frame
         // lands the bar at the bottom edge (SourceRectHeight - bar height) and the
-        // top corner clears back to backdrop.
+        // top corner clears back to background.
         position = SocialBarPosition.Bottom;
         VideoFrame? flipped = null;
         var deadline = DateTime.UtcNow.AddSeconds(5);