feat(chat-overlay): selection highlight + default size + starting backdrop

- Source.IsDraggable now includes ChatBox and WebSource (was Image-only)
- ChatBox default: 400×1080, top-right corner (1520, 0) on first add
- SeedStartingBackdrop: embeds starting-backdrop.jpg as Background source
  in the Starting scene on first launch (pack:// resource)

ViewModels/MainViewModel.cs

@@ -1351,8 +1351,11 @@ public class MainViewModel : ViewModelBase
                 Scenes.Add(scene);
 
             if (Scenes.Count == 0)
+            {
                 foreach (var name in SceneCatalog.All)
                     AddScene(name);
+                SeedStartingBackdrop();
+            }
 
             // The live backdrop belongs to Live only; a pre-policy DB may have
             // backdrops lingering in other scenes — drop them, then
@@ -1733,6 +1736,32 @@ public class MainViewModel : ViewModelBase
         ActiveScene = scene;
     }
 
+    private void SeedStartingBackdrop()
+    {
+        try
+        {
+            var uri = new Uri("pack://application:,,,/Assets/starting-backdrop.jpg", UriKind.Absolute);
+            using var stream = Application.GetResourceStream(uri)?.Stream;
+            if (stream == null) return;
+            using var ms = new MemoryStream();
+            stream.CopyTo(ms);
+            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",
+                Type = SourceType.Background,
+                AssetId = assetId,
+                Width = 1920,
+                Height = 1080,
+            });
+        }
+        catch { /* non-critical — stream works fine without a default bg */ }
+    }
+
     private void BeginEditElement(SceneElement? element)
     {
         if (element == null) return;
@@ -1790,7 +1819,15 @@ public class MainViewModel : ViewModelBase
             return;
         }
 
-        scene.Elements.Add(new Source { Name = NextSourceName(scene, baseName), Type = sourceType });
+        scene.Elements.Add(new Source
+        {
+            Name = NextSourceName(scene, baseName),
+            Type = sourceType,
+            X = sourceType == SourceType.ChatBox ? 1520 : 0,
+            Y = 0,
+            Width = sourceType == SourceType.ChatBox ? 400 : 0,
+            Height = sourceType == SourceType.ChatBox ? 1080 : 0,
+        });
         OnPropertyChanged(nameof(ShowEmptySceneHint));
         OnPropertyChanged(nameof(ShowSourcesEmptyHint));
         UpdateActiveBackground();