Add the step-by-step fixer and library types

Each pair is a Music (Artist/Album/01 Title) or Soundtracks (Album/01 Title)
library. The new Fixer tab takes the master, then the slave, through four
reviewed steps: folder structure, album tags (optional MusicBrainz lookup;
the slave reuses the fixed master's tags), folder names and file names,
then links the two libraries. Every fix is an undoable operation.

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

internal/engine/ops.go

@@ -635,6 +635,17 @@ func (e *Engine) Undo(ctx context.Context, opID int64) error {
 		e.log.Info("folder cover undone", "op", o.ID)
 		return nil
 	}
+	if o.Action == store.ActMoveDir {
+		if err := undoMoveDir(l, o); err != nil {
+			return err
+		}
+		if err := e.st.MarkUndone(ctx, o.ID); err != nil {
+			return err
+		}
+		e.log.Info("folder move undone", "op", o.ID, "folder", o.RelNew)
+		_, _ = e.StartScan(o.PairID, "undo")
+		return nil
+	}
 	if o.Action == store.ActMove {
 		if err := undoMove(l, o); err != nil {
 			return err
@@ -879,6 +890,13 @@ func (e *Engine) Recover(ctx context.Context) {
 		return
 	}
 	for _, o := range ops {
+		if o.Action == store.ActMoveDir {
+			if l, err := e.loadPair(ctx, o.PairID); err == nil {
+				recoverMoveDir(l, o, e.log)
+			}
+			_ = e.st.FinishOp(ctx, o.ID, store.OpInterrupted, "interrupted by shutdown; rescan to verify state")
+			continue
+		}
 		if o.Action == store.ActMove {
 			if l, err := e.loadPair(ctx, o.PairID); err == nil {
 				recoverMove(l, o)