Separate second-factor verification and delivery from auth (#40)

frontend/packages/sxchnge_auth/lib/src/dto/backend.dart

@@ -13,6 +13,8 @@ class BackendDTO {
       '';
   String get pendingToken =>
       (data['pendingToken'] as Map<String, dynamic>)['token'] as String;
+  String get challengeId => data['challengeId'] as String? ?? '';
+  String get channel => data['channel'] as String? ?? '';
   String get target => data['target'] as String;
   DateTime get expiresAt => DateTime.parse(data['expiresAt'] as String);
   DateTime get resendAt => DateTime.parse(data['resendAt'] as String);