Add terminal token endpoint and WebSocket auth
Add lexicon, pkl and generated types plus an xrpc handler to issue
short-lived terminal JWTs
Cloudflare sandbox WS now accepts token query param "t", validates the
JWT and checks ownership (added leftJoin to users)
Web client adds a hook and API call to fetch the token and append it to
the terminal WebSocket URL
Minor cleanups: replace several let with const and fix a createSandbox
variable assignment