Replaces naive auto-advance polling with full orchestrator matching
desktop behavior:
Gapless: when player signals gapless_ready (5s before track end),
downloads next track and calls play_next() for seamless transition.
Handles repeat-one (queue same track), defensive same-track check,
and upcoming queue peek (same as desktop's setGaplessGetNextTrackId).
Auto-advance: on track end, advances queue respecting repeat mode.
Repeat-one replays same track. Repeat-all wraps queue. Off stops
at queue end.
Shared download_track() helper used by play-track, play-album, and
orchestrator to avoid code duplication.
cc
vicrodh
39142aad3b