fix: BRB backdrop seed replaces existing instead of skipping

SeedBrbBackdrop() now updates the AssetId on an existing Background
source instead of bailing when one already exists. This ensures the
new brb-backdrop.jpg replaces any old image already in the DB.

ViewModels/MainViewModel.cs

@@ -1771,7 +1771,6 @@ public class MainViewModel : ViewModelBase
         {
             var brb = Scenes.FirstOrDefault(s => SceneCatalog.Is(s.Name, "BRB"));
             if (brb == null) return;
-            if (brb.Elements.OfType<Source>().Any(s => s.Type == SourceType.Background)) return;
 
             var uri = new Uri("pack://application:,,,/Assets/brb-backdrop.jpg", UriKind.Absolute);
             using var stream = Application.GetResourceStream(uri)?.Stream;
@@ -1781,14 +1780,25 @@ public class MainViewModel : ViewModelBase
             var bytes = ms.ToArray();
             var assetId = AddAsset(bytes);
             if (assetId == null) return;
-            brb.Elements.Add(new Source
+
+            var existing = brb.Elements.OfType<Source>().FirstOrDefault(s => s.Type == SourceType.Background);
+            if (existing != null)
             {
-                Name = "Background",
+                existing.AssetId = assetId;
-                Type = SourceType.Background,
+                existing.Width = 1920;
-                AssetId = assetId,
+                existing.Height = 1080;
-                Width = 1920,
+            }
-                Height = 1080,
+            else
-            });
+            {
+                brb.Elements.Add(new Source
+                {
+                    Name = "Background",
+                    Type = SourceType.Background,
+                    AssetId = assetId,
+                    Width = 1920,
+                    Height = 1080,
+                });
+            }
         }
         catch { /* non-critical — stream works fine without a default bg */ }
     }