add stereo support

[afraser]

getDisplayMedia capture is stereo, but WebRTC's Opus codec defaults to mono.

Looking at createPeerConnection in HostView.jsx:

  const offer = await pc.createOffer();                         
  await pc.setLocalDescription(offer);                                                                                                                                                            

The generated SDP will have an Opus fmtp line like:
a=fmtp:111 minptime=10;useinbandfec=1
No stereo=1 → Opus encodes 1 channel. The source audio gets down-mixed before it ever leaves the host.

To get stereo you'd need to mangle the SDP before setting it as the local description.