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.
4ec9b936a6