Resolve channel links before mounting a document workspace

A /channels/:id URL used to mount a room, canonicalize to the document path,
then mount a second room under the document host. The channel route now only
resolves the document path (showing Loading or a Failure) and the document
host mounts once under its canonical route identity, so child open and close
keep the same parent room.

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