fix(background): refresh BOTH preview layers, not just the thumbnails

The preview draws two background images — BackgroundImage and, above it, ActiveBackgroundImage
(the static art) — while the mini-pane reads the scene snapshot. The default-restore path only
called UpdateBackgroundImage, so the thumbnails updated and the preview did not.

It now calls UpdateActiveBackground as well.

Also adds the generic cure behind the Background bug: OnSelectedElementPropertyChanged invalidates
the scene bake for ANY property change. A static layer's pixels are cached by element identity, so
a property edit without invalidation repaints nothing and the control looks dead — the failure was
never specific to UseDefaultBackground, only first noticed there.

Background + thumbnail tests pass (21); Release 0 warnings.

ViewModels/MainViewModel.cs

@@ -105,6 +105,13 @@ public partial class MainViewModel : ViewModelBase
 
     private void OnSelectedElementPropertyChanged(object? sender, System.ComponentModel.PropertyChangedEventArgs e)
     {
+        // ⛔ Safety net for EVERY property control: a static layer's appearance is baked into the
+        // scene cache keyed on element identity, so changing a property without invalidating the
+        // bake repaints nothing — the control looks dead. The Background toggle was the first case
+        // found (2026-11-01); the cause is generic, so the cure is here rather than one handler per
+        // property. Cheap: the bake is recomputed on demand, once.
+        if (StagedScene != null) _sceneGraph.InvalidateBake(StagedScene);
+
         if (e.PropertyName == nameof(SceneElement.UseDefaultBackground))
         {
             // ⛔ Turning the default back ON has to RE-STAMP the shipped art. Browse… writes the
@@ -122,8 +129,12 @@ public partial class MainViewModel : ViewModelBase
                 }
             }
 
-            if (StagedScene != null) _sceneGraph.InvalidateBake(StagedScene);
+            // Both surfaces: UpdateBackgroundImage drives the preview's lower layer,
+            // UpdateActiveBackground drives ActiveBackgroundImage (the art the preview draws above
+            // it). Refreshing only the first is why the thumbnails changed while the preview did
+            // not (creator report, 2026-11-01).
             UpdateBackgroundImage();
+            UpdateActiveBackground();
             RefreshSnapshotsAsync();
             ScheduleSave();
         }