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

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

@@ -5,6 +5,13 @@ import '../models/two_factor.dart';
 import '../models/verification.dart';
 
 abstract final class BackendMapper {
+  static VerificationResult activation(BackendDTO dto) {
+    if (!dto.verified) {
+      throw const VerificationException(VerificationError.internal);
+    }
+    return VerificationResult(isSimulated: false, verifiedEmail: dto.email);
+  }
+
   static AuthResult signup(BackendDTO dto) {
     if (dto.userId.isEmpty) throw const FormatException('Missing user');
     return AuthResult(userId: dto.userId, isSimulated: false);