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

Services/ScreenCaptureFrameSource.cs

@@ -30,10 +30,10 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
     private bool _framePending;
     private DateTime _lastErrorLog = DateTime.MinValue;
 
-    // The composition master frame (see ai.md "Resolution tiers"): the backdrop
+    // The composition master frame (see ai.md "Resolution tiers"): the background
     // is an input layer, so we never hold a CPU frame bigger than the master.
-    private const int MaxBackdropWidth = 1920;
-    private const int MaxBackdropHeight = 1080;
+    private const int MaxBackgroundWidth = 1920;
+    private const int MaxBackgroundHeight = 1080;
 
     // A failing conversion must not re-flood the log at frame rate.
     private static readonly TimeSpan ErrorLogThrottle = TimeSpan.FromSeconds(5);
@@ -172,9 +172,9 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
         // The master frame is 1920×1080; a larger monitor is scaled down here so
         // the CPU never holds a frame above the master (the rendered layer fills
         // the frame with UniformToFill regardless).
-        if (sw > MaxBackdropWidth || sh > MaxBackdropHeight)
+        if (sw > MaxBackgroundWidth || sh > MaxBackgroundHeight)
         {
-            var scale = Math.Min(MaxBackdropWidth / (double)sw, MaxBackdropHeight / (double)sh);
+            var scale = Math.Min(MaxBackgroundWidth / (double)sw, MaxBackgroundHeight / (double)sh);
             var dw = Math.Max(1, (int)(sw * scale));
             var dh = Math.Max(1, (int)(sh * scale));
             return new VideoFrame(dw, dh, DownscaleBgra(data, sw, sh, srcStride, dw, dh));