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/SceneElement.cs

@@ -47,6 +47,9 @@ public abstract class SceneElement : INotifyPropertyChanged
     /// <summary>True only for singleton resources (webcam). Gates webcam-only UI.</summary>
     public virtual bool IsWebcam => false;
 
+    /// <summary>True for elements that can be dragged/resized in the preview (image, webcam).</summary>
+    public virtual bool IsDraggable => false;
+
     /// <summary>True only for image sources — the raw preview renderer's discriminator.</summary>
     public virtual bool IsImageSource => false;