Set up the workspace and the room connection

A Bun workspace with three packages and two apps, and enough of the wire to
prove the shape works: a WebSocket carrying one room, identity asserted at the
upgrade, and presence relayed to everyone already there.

Identity is a claimed GitHub handle, stored per tab rather than per origin so
two tabs are two people. The socket URL is derived from `location` and never
configured, so localhost, a LAN address and a tunnel all work from one build;
Vite proxies `/ws` in development so the browser sees a single origin in either
mode.

A browser cannot read the status of a refused upgrade, so a connection that
never opened asks the same endpoint over HTTP to find out why. Otherwise a
rejected access key is indistinguishable from an unreachable server, and the
client would back off against a door that will not open.

Lexical, Yjs and React are deduped in Vite up front: two copies of Lexical
throw `incompatible editors` and every node operation fails quietly afterwards.