Destroy the mpv instance before the track-change router.replace in
replaceWithTrackSelection and goToPreviousItem, matching goToNextItem — two
overlapping decoders/surfaces during the transition OOM low-RAM devices
(CodeRabbit). addSubtitleFile now sets the live index to the downloaded local
sub's real index (LOCAL_SUBTITLE_INDEX_START - position) instead of a blanket
-100, which collided with the first local sub and mis-recorded the selection
(Copilot); it still resolves to notFound on the onTracksReady re-apply so it
doesn't clobber the freshly selected track.
direct-player resolves the selection on onTracksReady (online + offline, init +
runtime), VideoContext does the same for the mobile menu (incl. offline-transcoded),
and the menus (SubtitleTrackSelector, VideoContext, TVSubtitleSheet) now order
tracks like jellyfin-web. Fixes wrong-subtitle/wrong-language selection.
Fixes#954
