fix: clear stale cover art and backgrounds on tracks without art (#957)

* fix: ensure load callback fires on error

Calls callOnLoaded(nil) when image loading fails.

This fixes the bottomplayer silently retaining stale images by ensuring consumers always get a callback and can clear their display state.

* fix: clear background layers on missing art

nil state now hides the blurred background widgets or animates the gradient to transparent, depending on the active configuration

ui/util/thumbnailloader.go

@@ -52,6 +52,7 @@ func (i *ThumbnailLoader) Load(coverID string) {
 		fyne.Do(func() {
 			if err != nil {
 				log.Printf("Error loading cover image: %s", err.Error())
+				i.callOnLoaded(nil)
 			} else {
 				i.callOnLoaded(img)
 			}