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/engine.go

@@ -43,6 +43,9 @@ type Engine struct {
 
 	beforeAction func(ctx context.Context, reason string) error
 
+	emptyMu sync.Mutex
+	empty   map[int64]*NoMusicScan // latest scan for folders without music, per pair
+
 	jobsMu sync.Mutex
 	jobs   map[int64]*Job
 	nextID int64