register page (350 lines): hero render when no codeError, NO_INVITE_CODE
vs other-codeError card branches, form hidden when codeError set,
back-to-login link, form section rendering, prefill hydration of
firstName/lastName/email, prefill-hint visibility branch, hidden
code input with code-null fallback.
admin/users/new: heading, three card sections, group checkboxes
rendered, form-error banner branch, cancel link, submit button.
17 tests across two pages.
Refs #496.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
c6f6822781