WebBridge: skip setMediaItems/prepare when the track is already buffered
from the cold-start pre-warm, so resuming the last song is instant (~0s)
instead of re-buffering from scratch (~10s).
Player: show pause icon during buffering (isPlaying=false, isBuffering=true)
so the button doesn't flash back to "play" between tracks. Also optimistically
set _nativePlaying=true and call setPlayIcon immediately on playCurrent so the
icon updates before the native callback round-trip.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>