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.Sources.cs

@@ -237,11 +237,11 @@ public partial class MainViewModel
     // session when the last using scene lets go). The webcam identity survives —
     // it's the app-level default, so removing a scene's placement re-enables Add
     // for the same camera instead of dropping it.
-    private void RemoveElement(SceneElement? element)
+    internal void RemoveElement(SceneElement? element)
     {
         var scene = StagedScene;
         if (scene == null || element == null) return;
-        if (element is Source { IsBackground: true }) return;
+        if (element is Source { IsBackground: true } or Source { IsBrandLogo: true }) return;
         if (element is WebcamSceneConfig && _webcam != null)
         {
             if (SelectedElement == element)