Add album art search, auto-match and a whole-album tag editor

Search iTunes, Deezer, MusicBrainz, DuckDuckGo and Google for square covers,
embed them and/or save cover.jpg, auto-match covers for albums, and edit an
album's shared tags and track list in one place.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/ops.go

@@ -625,6 +625,16 @@ func (e *Engine) Undo(ctx context.Context, opID int64) error {
 	if err != nil {
 		return err
 	}
+	if o.Action == store.ActCover {
+		if err := e.undoCover(l, o); err != nil {
+			return err
+		}
+		if err := e.st.MarkUndone(ctx, o.ID); err != nil {
+			return err
+		}
+		e.log.Info("folder cover undone", "op", o.ID)
+		return nil
+	}
 	if o.Action == store.ActMove {
 		if err := undoMove(l, o); err != nil {
 			return err