Sign in with Google, or null. Use Google Identity Services
(https://accounts.google.com/gsi/client) with client_id: clientId; its callback's
credential is the id_token.
Sign in with Klarna, or null. Pass clientId and environment to Klarna's Web SDK
(https://js.klarna.com/web-sdk/v1/klarna.js) and scope to its identity button, then send
signin.user_account_linking.user_account_linking_id_token to auth.signInWithProvider("klarna", …).
Magic link / emailed code sign-in. Always true: it confirms the email and recovers lost passwords.
Sign in with Microsoft, or null. Run an auth code + PKCE sign-in against authority with
clientId and scope (e.g. with MSAL: loginPopup / loginRedirect) and send the resulting
idToken. accounts says which accounts the store accepts.
Email + password sign-in. false when the store chose "Magic link only".
Whether self-registration is open, per customer type.
GET /auth/methods: which sign-in options the store has turned on.