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/store/models.go

@@ -653,6 +653,8 @@ const (
 	ActBuild   = "build"   // master file linked into the slave (fresh library)
 	ActRemove  = "remove"  // slave hardlink removed (master copy kept)
 	ActRename  = "rename"  // slave folder renamed to the master's spelling
+
+	ActRemoveFolder = "remove-folder" // folder without music moved to the quarantine
 )
 
 // Operation statuses.