audio: mobile — back button closes the now-playing sheet (history pushState/popstate)
On mobile, openSheet pushes a history entry; the phone's back button (popstate)
then closes the sheet first instead of navigating away. UI close (X/swipe/backdrop/
Esc) pops the entry itself via history.back() so the balance stays correct. Gated
on isMobileView() (desktop unchanged). audio-player.js ?v=7 -> ?v=8.
Co-Authored-By: Claude <noreply@…>