[Hellraiser28]

Fixes #61.

With MUSIC_FOLDERS=/music,/audiobooks the root folders have a single path segment. The song list requested /api/songs/music, and Next.js routes a single segment to /api/songs/[id] before the optional catch-all /api/songs/[[...path]]. That route answers 400 Invalid song ID, so the list stayed empty and the header showed ? files. A subfolder named like a song sub-route had the same problem, e.g. /music/audio went to /api/songs/[id]/audio.

The list now sends the folder as a folderPath query parameter, the way the adjacent and bulk endpoints already receive it. The route reads folderPath first and still accepts the folder as path segments.

Tested

• route.test.ts with 4 new tests; all 329 tests pass. ESLint, Prettier and tsc are clean on the changed files (the unused SongColumnFilters import warning in route.ts is already on main).
• Docker build of this branch with MUSIC_FOLDERS=/music,/audiobooks, files directly in /music and a subfolder /music/audio:
  • v1.15.0: /music, /audiobooks and /music/audio return 400 Invalid song ID.
  • This branch: /music lists its 7 songs, /music/audio 1, /music/Albums/Amrinder Gill/Judaa 3, All Folders 13.
  • In the browser, clicking music and audio shows the songs and the file count.