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/Compositor/SceneCompositor.cs

@@ -9,14 +9,14 @@ namespace ytLive.Services.Compositor;
 /// editing view; this is the output view — see TASKS.md "Ship step 1 — Scene compositor".
 ///
 /// Frame sources are supplied by a <see cref="Func{SceneElement, VideoFrame}"/> resolver
-/// (the caller maps webcam → DeviceId, images → AssetId, backdrop → CaptureKey), keeping
+/// (the caller maps webcam → DeviceId, images → AssetId, background → CaptureKey), keeping
 /// the compositor pure and free of WPF and of the capture managers.
 /// </summary>
 public sealed class SceneCompositor
 {
     /// <summary>
     /// Composite <paramref name="scene"/> into the tier's output frame. Layer order (back →
-    /// front): live backdrop (the scene's <c>IsBackdrop</c> source) → background image →
+    /// front): live background (the scene's <c>IsBackground</c> source) → background image →
     /// visible elements (z-order = <c>Elements</c> order, mirroring the XAML DataTemplate) →
     /// branding flash → social bar (a global overlay; spans the master width at
     /// <paramref name="socialBarTop"/>). Transparent regions read opaque black.
@@ -45,10 +45,10 @@ public sealed class SceneCompositor
 
         var elements = scene.Elements;
 
-        var backdrop = elements.OfType<Source>().FirstOrDefault(s => s.IsBackdrop);
-        var backdropFrame = backdrop != null && backdrop.IsVisible ? frameFor(backdrop) : null;
-        if (backdropFrame != null)
-            BlitContent(buffer, cropW, cropH, 0, 0, cropW, cropH, backdropFrame, 1f, false, false);
+        var liveCapture = elements.OfType<Source>().FirstOrDefault(s => s.IsBackground);
+        var liveCaptureFrame = liveCapture != null && liveCapture.IsVisible ? frameFor(liveCapture) : null;
+        if (liveCaptureFrame != null)
+            BlitContent(buffer, cropW, cropH, 0, 0, cropW, cropH, liveCaptureFrame, 1f, false, false);
 
         var background = elements.OfType<Source>().FirstOrDefault(s => s.Type == SourceType.Background);
         var backgroundFrame = background != null ? frameFor(background) : null;
@@ -60,10 +60,10 @@ public sealed class SceneCompositor
             if (!element.IsVisible) continue;
             switch (element)
             {
-                case Source { IsBackdrop: true }:
+                case Source { IsBackground: true }:
                 case Source { Type: SourceType.Background }:
                 case Source { Type: SourceType.TextOverlay }:
-                    continue; // backdrop/background are their own layers; Text isn't shipped
+                    continue; // background/background are their own layers; Text isn't shipped
             }
 
             var frame = frameFor(element);
@@ -91,6 +91,34 @@ public sealed class SceneCompositor
             new VideoFrame(cropW, cropH, buffer), options.OutputWidth, options.OutputHeight);
     }
 
+    /// <summary>
+    /// Render a cross-fade blend between two scenes. Used by <see cref="TransitionService"/>
+    /// during fade/move transitions: the caller renders each scene individually, then blends
+    /// the two results at the given alpha (0 = fully from, 1 = fully to).
+    /// </summary>
+    public static VideoFrame BlendFrames(VideoFrame from, VideoFrame to, float alpha)
+    {
+        if (from.Width != to.Width || from.Height != to.Height)
+        {
+            if (alpha <= 0.5f) return from;
+            return to;
+        }
+
+        var count = from.BgraPixels.Length;
+        var result = new byte[count];
+        var inv = 1f - alpha;
+
+        for (var i = 0; i < count; i += 4)
+        {
+            result[i]     = (byte)(from.BgraPixels[i]     * inv + to.BgraPixels[i]     * alpha);
+            result[i + 1] = (byte)(from.BgraPixels[i + 1] * inv + to.BgraPixels[i + 1] * alpha);
+            result[i + 2] = (byte)(from.BgraPixels[i + 2] * inv + to.BgraPixels[i + 2] * alpha);
+            result[i + 3] = 255;
+        }
+
+        return new VideoFrame(from.Width, from.Height, result);
+    }
+
     /// <summary>
     /// UniformToFill blit of a source frame into element rect (ex, ey, ew, eh), with
     /// straight-alpha source-over, optional round clip (true circle, hard edge) and