Sign-in method unavailable

The way to sign in can't be added here (the account API,): the brand doesn't offer it, it can't show a fresh sign-in, or the account has one of it already (an email address).

Field	Value
`code`	`sign_in_method_unavailable`
`status`	409 Conflict
`title`	Sign-in method unavailable

Synq's APIs answer errors with a problem document (RFC 9457, application/problem+json). Its type is the address of this page, and its code is sign_in_method_unavailable. Branch on code, never on title or detail: a code never changes once it ships, while the words may.

{
  "type": "…/errors/sign_in_method_unavailable",
  "title": "Sign-in method unavailable",
  "status": 409,
  "code": "sign_in_method_unavailable"
}

See every error code.