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.

ViewModels/MainViewModel.Background.cs

@@ -438,6 +438,7 @@ public partial class MainViewModel
                 StampDefaultBackgroundAsset(background, scene.Name);
         }
         SeedOnAirImage();
+        EnsureBrandLogos();
         OnPropertyChanged(nameof(ShowSourcesEmptyHint));
         OnPropertyChanged(nameof(LayerCount));
     }