creator feedback batch: 9 fixes from initial review

1. About box bg → #0A0C1A (matches icon bg)
2. Elements panel collapses when no element selected / scene changes
3. OpacityChip moved from preview overlay into Elements panel
4. Color picker fix: cast to SceneElement not Source (WebcamSceneConfig was silently blocked)
5. Red left accent bar on selected items in Scene/Source lists
6. ✓ character replaced with Path checkmark icon in Elements panel
7. Removed ToolTip='Applied' from green check buttons
8. Created bugs.md with first entry (iTunes audio on mic meter)
9. Full logo in README + About box uses ytLlive-logo.png

Models/Source.cs

@@ -42,6 +42,8 @@ public class Source : SceneElement
         }
     }
 
+    public override bool IsDraggable => Type == SourceType.Image;
+
     /// <summary>The permanent scene backdrop (live desktop/game capture). Never removable/reorderable.</summary>
     private bool _isBackdrop;
     public bool IsBackdrop { get => _isBackdrop; set => Set(ref _isBackdrop, value); }