Prefetch the sign-in page and keep stale routes from canonicalizing

- Start loading the lazy GitHub sign-in chunk beside the session request so a
  signed-out visit does not wait on a second round trip.
- An outgoing document route that resolves while a newer route is still
  loading no longer publishes its canonical path over the newer request. The
  race predates the lazy /channels chunk, which widened it (flaky "adds a
  Project and creates its first document").
- Explain why ChannelWorkspace receives Failure, Loading and retryable as props.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>