Multi-scene webcam (schema v3/v4): singleton Webcam + per-scene WebcamSceneConfig, right-click OBS-style border/context menu, persisted round-to-rect restore + legacy-square 16:9 heal, dark MenuItem template, tests (25 passing)

Models/Scene.cs

@@ -30,7 +30,15 @@ public class Scene : INotifyPropertyChanged
         set => Set(ref _isHidden, value);
     }
 
-    public ObservableCollection<Source> Sources { get; } = new();
+    /// <summary>
+    /// The scene's rendered elements in z-order (back to front): multi-instance
+    /// Sources plus this scene's webcam usage (<see cref="WebcamConfig"/>), if any.
+    /// </summary>
+    public ObservableCollection<SceneElement> Elements { get; } = new();
+
+    /// <summary>This scene's webcam usage, or null if the creator hasn't added the webcam here.</summary>
+    public WebcamSceneConfig? WebcamConfig => Elements.OfType<WebcamSceneConfig>().FirstOrDefault();
+
     public bool IsChatScene { get; init; }
 
     public event PropertyChangedEventHandler? PropertyChanged;