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)

Models/Source.cs

@@ -44,7 +44,7 @@ public class Source : SceneElement
         }
     }
 
-    public override bool IsDraggable => Type == SourceType.Image;
+    public override bool IsDraggable => Type is SourceType.Image or SourceType.WebSource or SourceType.ChatBox;
 
     public override bool IsWebSource => Type == SourceType.WebSource;