getCoverArt now falls back to sidecar images next to the first track in
an album, so browse responses can advertise a coverArt id whether or not
albums.cover_art_path is set. Worst case the client gets a Subsonic 70
when no sidecar exists.
Adds the Subsonic browse wire types (MusicFolder, Index, ArtistRef,
ArtistDetail, AlbumRef, AlbumDetail, SongRef, and their response
envelopes) with dual json+xml tags so a single struct serves both
formats. Includes UUID wire helpers (uuidToID/parseUUID), indexing
helpers (indexLetter), MIME mapping (contentTypeForFormat), and the
artist-name resolver used by album listings.
IDs are bare UUID strings on the wire — endpoint context disambiguates.
bvandeusen