Add a ReorderableSongList composable that supports long-press drag-and-drop
reordering with visual drop target highlighting. Drag-and-drop is only
enabled when songs.order is "manual"; alphabetical mode shows a hint.
The custom order is passed to SongbookPipeline via a new customSongOrder
parameter and a reset button restores the original order.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
