Complete verification links and preserve verified login email (#60)

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

@@ -6,6 +6,8 @@ class BackendDTO {
     (jsonDecode(body) as Map<String, dynamic>)['data'] as Map<String, dynamic>,
   );
   final Map<String, dynamic> data;
+  bool get verified => data['verified'] == true;
+  String? get email => data['email'] as String?;
   String get error => data['error'] as String? ?? 'unavailable';
   String get userId =>
       data['userId'] as String? ??