fix: seed starting backdrop on every load, not just first launch

SeedStartingBackdrop() moved outside the Scenes.Count==0 guard so it
fires on every startup. Existing layouts with a Background source are
skipped (idempotent check).

ViewModels/MainViewModel.cs

@@ -1354,8 +1354,8 @@ public class MainViewModel : ViewModelBase
             {
                 foreach (var name in SceneCatalog.All)
                     AddScene(name);
-                SeedStartingBackdrop();
             }
+            SeedStartingBackdrop();
 
             // The live backdrop belongs to Live only; a pre-policy DB may have
             // backdrops lingering in other scenes — drop them, then
@@ -1740,6 +1740,10 @@ public class MainViewModel : ViewModelBase
     {
         try
         {
+            var starting = Scenes.FirstOrDefault(s => SceneCatalog.Is(s.Name, "Starting"));
+            if (starting == null) return;
+            if (starting.Elements.OfType<Source>().Any(s => s.Type == SourceType.Background)) return;
+
             var uri = new Uri("pack://application:,,,/Assets/starting-backdrop.jpg", UriKind.Absolute);
             using var stream = Application.GetResourceStream(uri)?.Stream;
             if (stream == null) return;
@@ -1748,8 +1752,6 @@ public class MainViewModel : ViewModelBase
             var bytes = ms.ToArray();
             var assetId = AddAsset(bytes);
             if (assetId == null) return;
-            var starting = Scenes.FirstOrDefault(s => SceneCatalog.Is(s.Name, "Starting"));
-            if (starting == null) return;
             starting.Elements.Add(new Source
             {
                 Name = "Background",