Add reusable two-factor verification for login and money operations (#32)

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

@@ -0,0 +1,20 @@
+import '../dto/two_factor.dart';
+import '../models/two_factor.dart';
+
+abstract final class FactorMapper {
+  static FactorSettings settings(FactorSettingsDTO dto) => FactorSettings(
+    enabled: dto.enabled,
+    target: dto.target,
+    method: FactorMethod.values.byName(dto.method),
+  );
+  static FactorChallenge challenge(FactorChallengeDTO dto) => FactorChallenge(
+    id: dto.id,
+    purpose: FactorPurpose.values.byName(dto.purpose),
+    operation: dto.operation,
+    target: dto.target,
+    expiresAt: dto.expiresAt,
+    resendAt: dto.resendAt,
+    method: FactorMethod.values.byName(dto.method),
+  );
+  static FactorGrant grant(FactorGrantDTO dto) => FactorGrant(dto.token);
+}