Brand logo: one locked image layer per scene above the background, image from App Settings

- Source.IsBrandLogo persisted (additive migration); layer healed on load, removal blocked
- Image stored as an asset; settings keep the asset id and file name
- App Settings: Brand Logo file field with Browse
- Default placement top-left; per-scene eye uses the existing element visibility
Test: BrandLogoTests; layout/background tests pass (34). Not pushed.

Models/Source.cs

@@ -88,6 +88,10 @@ public class Source : SceneElement
     public override bool IsWebSource => Type == SourceType.WebSource;
     public override bool IsTextOverlay => Type == SourceType.TextOverlay;
 
+    /// <summary>The permanent brand logo layer (one per scene, directly above the background).
+    /// Locked against removal; its image comes from the App Settings brand logo.</summary>
+    public bool IsBrandLogo { get; set; }
+
     /// <summary>The permanent scene background (live desktop/game capture). Never removable/reorderable.</summary>
     private bool _isBackground;
     public override bool IsBackground { get => _isBackground; set => Set(ref _isBackground, value); }