Files with multiple embedded tracks (e.g. MKV from media sources or
downloads) previously exposed none of them: mpv's track-list was never
read, aid was never set, and sid was only ever no/auto. The quality
selector showed no Audio or Subtitles tab for such files.
MPVClient now observes track-list/aid/sid and delivers a coalesced,
leniently decoded [MPVTrack] snapshot; selection state is derived from
the reported "selected" flags. Embedded tracks are switched live via
aid/sid (no reload), surfaced in the existing Audio/Subtitles sections
alongside external captions on iOS, tvOS and macOS. External sub-add/
audio-add tracks are filtered out to avoid double-listing. User picks
are sticky across same-video reloads (quality switch, audio mode,
retries) and preferred audio/subtitle languages auto-select a matching
embedded track once per load, with external captions taking precedence.
Hide the toggle on macOS, where apps are never suspended and playback
always continues regardless of the setting. Pause playback on iOS when
backgrounding with the setting disabled — the audio background mode
previously kept mpv playing, making the off state a no-op.
Move the continue-playing policy into PlayerService and make the
backend's background render pause unconditional (PiP-gated only):
it is a GPU optimization that should not depend on the setting.
Also ungate the player sheet visibility handlers, which manage
foreground mini player rendering unrelated to background playback.
