LikedIdsController loads /api/likes/ids once and mutates the local set
on toggle. Failed mutations roll the set back so the icon never lies.
Wired into ArtistDetail header, AlbumDetail header, and per-track in
the album track list.
Headers carry placeholder play buttons (wired in Task 18) + room for
LikeButton (Task 16). Artist screen has a 2-column album grid; album
screen has a track list with mm:ss durations. Routes :id-parameterized
under the shell so the PlayerBar persists.
bvandeusen