setSignedIn method
로그인 성공으로 설정
Implementation
void setSignedIn(KAuthUser user, {String? serverToken, DateTime? expiresAt}) {
mockUser = user;
mockFailure = null;
_currentUser = user;
_serverToken = serverToken;
_expiresAt = expiresAt ?? DateTime.now().add(const Duration(hours: 1));
_initialized = true;
_authStateController.add(user);
}