Add finding and removing folders without music

The new Cleanup tab lists the top-most folders in either library that
contain no music file (per the pair's extensions). Removing them needs
confirmation, plus an explicit acknowledgement when they still hold
files; each folder is re-checked for music and moved into the
quarantine (undoable from History, purged after the retention period).

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

internal/engine/ops.go

@@ -622,7 +622,7 @@ func (e *Engine) Undo(ctx context.Context, opID int64) error {
 		return errors.New("the quarantined original was already purged")
 	}
 	switch o.Action {
-	case store.ActLink, store.ActPromote, store.ActBuild, store.ActRemove, store.ActRename:
+	case store.ActLink, store.ActPromote, store.ActBuild, store.ActRemove, store.ActRename, store.ActRemoveFolder:
 	default:
 		// History can hold operations of features that were removed (tag
 		// edits, file moves); undoing them as a link would be wrong.
@@ -632,6 +632,17 @@ func (e *Engine) Undo(ctx context.Context, opID int64) error {
 	if err != nil {
 		return err
 	}
+	if o.Action == store.ActRemoveFolder {
+		if err := undoRemoveFolder(l, o); err != nil {
+			return err
+		}
+		if err := e.st.MarkUndone(ctx, o.ID); err != nil {
+			return err
+		}
+		e.log.Info("folder restored from quarantine", "op", o.ID)
+		pruneEmpty(filepath.Dir(o.QuarantinePath), l.quarantine)
+		return nil
+	}
 	if o.Action == store.ActRename {
 		if err := undoRename(l, o); err != nil {
 			return err