Stream the response

My model call waits for the whole response before showing anything,
and it feels broken: <paste the endpoint and the client code>

Add streaming end to end:
- Server: stream from the provider and forward to the client without
  buffering the whole thing.
- Client: render tokens as they arrive, scrolling only if the user is
  already at the bottom.
- A stop button that actually aborts the upstream request, so I'm not
  billed for output nobody sees.
- Handle the stream failing halfway: keep what arrived, show a clear
  error, offer retry.
- Handle the user navigating away mid-stream.

Tell me what changes for error handling, since headers are sent before
the model has finished.