loginstringrequired
A user's login, which is their namespace, or an organization's (one space for both): 1 to 39 lower-case letters, digits and single hyphens, no hyphen at either end (input in any case is lowered). Reserved, because a namespace is the first URL segment: v1, healthz, login, logout, settings, assets, fonts, new, api, app, static, orgs, users, docs, install. A login made at sign-in comes from the email's local part, skipping reserved and taken names (base, base-2, ... base-999, then a random tail).
pattern: ^[a-z0-9](?:[a-z0-9]|-(?=[a-z0-9])){0,38}$
