feat(config): show credential setup form when enabling require_login

[javi11]

Summary

• When the Require Login toggle is turned ON in the Auth Config section and no users exist in the database, a Set Up Admin Credentials form now appears inline — preventing admin lockout on first enable
• Form collects username (min 3 chars), password (min 8 chars), and confirm password with client-side validation
• On save, the component registers the user via POST /api/auth/register before saving the config — both steps happen atomically from the UI's perspective
• When login is required and a user already exists, a green Credentials configured badge is shown instead (with a pointer to the user menu for password changes)
• When login is disabled, the existing security warning continues to appear as before

What changed

frontend/src/components/config/AuthConfigSection.tsx — only file changed; no backend changes needed (existing /api/auth/register and /api/auth/registration-status endpoints cover the requirements.

• Fetches registration status on mount via apiClient.checkRegistrationStatus()
• New local state: registrationStatus, credentialForm, credentialError, isRegistering
• handleSave now runs registration before config save when credentials are needed
• Two new conditional UI blocks: credential form (no users) and status badge (users exist)

Test plan

• Toggle Require Login → ON with no existing users → credential form appears
• Submit without filling form → validation error shown, save blocked
• Fill valid credentials and save → user registered, config saved, redirected to login
• Log in with the new credentials successfully
• With users already present, toggle Require Login → ON → "Credentials configured" badge shown, no form
• Toggle Require Login → OFF → security warning shown, no credential form