useEventsDispatcher subscribes to player.current/state via \$effect:
on first transition to playing for a new track, POSTs play_started
and caches the id; on track-id change while a play is open, closes
the prior row as play_skipped; on natural completion (state
playing→paused with position >= duration > 0), POSTs play_ended;
on pagehide, navigator.sendBeacon fires a final play_skipped.
Stable per-tab client_id from sessionStorage.
