Library api

[sb-3000]

Library API: folder artwork opt-in, request validation, natural sort, by-columns grouping

• Folder artwork now follows standard resolution (first track's art via album_art_manager);
  the folder image lookup is opt-in with folderImage=true
• 400 when adding with neither items nor query (previously added the whole library);
  use items: [""] to add everything explicitly
• 400 for a bare range=N on library endpoints; the playlists API is unchanged
• Natural, case-insensitive ordering in by-path via pfc::sysNaturalSortCompareI
• GET /api/library/items/by-columns: one Album List-style pattern