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/WebcamSceneConfig.cs

@@ -0,0 +1,16 @@
+using System.Windows.Media;
+
+namespace ytLive.Models;
+
+/// <summary>
+/// A scene's usage of the app-wide webcam — conceptually `webcam.{scene}.config`.
+/// The identity lives once on the <see cref="Webcam"/> entity; this object is that
+/// scene's placement, clip/mirror, border, and visibility for it. Exists only in
+/// scenes where the creator added the webcam.
+/// </summary>
+public class WebcamSceneConfig : SceneElement
+{
+    public string WebcamId { get; init; } = string.Empty;
+    public override bool IsWebcam => true;
+    public override ImageSource? DisplaySource => VideoImageSource;
+}