defaultUserStore

A UserStore over the local preferences: one person, this browser.

Attribution needs *some* answer even when an app has no directory to ask — a solo board still wants "you" on the shapes you made. This is that answer, and it is entirely local: no service, no lookup, nothing leaves the page. An unknown id resolves to null rather than to a placeholder, so a UI can tell "somebody I cannot name" from "nobody".