Folder Library now uses the standard data-fn/data-v3-play markup so the song_preview plugin handles hover-previews, the same way it does for grid and list views. Removes ~200 lines of preview-specific code: dedicated audio element, indicator rendering, toolbar toggle, and backend manifest checking. The preview experience is now unified across all library surfaces.
Signed-off-by: Kyle <kyle.j.t@live.co.uk>