@spec resolve_link_conflict(map(), Gamend.Accounts.User.t() | nil) ::
  {Gamend.Accounts.User.t() | nil, String.t() | nil}

The pending OAuth link conflict for this session, as {conflict_user, provider}.

A conflict exists only when GamendWeb.AuthController proved one: the caller completed a provider callback and that provider identity was already linked to another account. It is recorded in the session there, so the id can never be supplied by the reader.

Re-validated on every mount, because the session outlives the situation: the other account may since have unlinked the provider, been deleted, or turned out to be the caller's own. Anything that no longer holds resolves to no conflict.