Integrate email notifications into signup and login (#38)

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

@@ -0,0 +1,45 @@
+import '../dto/backend.dart';
+import '../models/auth.dart';
+import '../models/session.dart';
+import '../models/two_factor.dart';
+
+abstract final class BackendMapper {
+  static AuthResult signup(BackendDTO dto) {
+    if (dto.userId.isEmpty) throw const FormatException('Missing user');
+    return AuthResult(userId: dto.userId, isSimulated: false);
+  }
+
+  static FactorChallenge challenge(BackendDTO dto, String id) =>
+      FactorChallenge(
+        id: id,
+        purpose: FactorPurpose.login,
+        operation: id,
+        target: dto.target,
+        expiresAt: dto.expiresAt,
+        resendAt: dto.resendAt,
+      );
+  static AuthSession session(BackendDTO dto) {
+    if (dto.accessToken.isEmpty || dto.userId.isEmpty) {
+      throw const FormatException('Missing session');
+    }
+    return AuthSession(dto.userId, dto.accessToken, dto.refreshToken);
+  }
+
+  static AuthException authFailure(int status, BackendDTO dto) =>
+      AuthException(switch (dto.error) {
+        'email_in_use' => AuthError.emailInUse,
+        'account_not_verified' => AuthError.accountNotVerified,
+        _ when status == 401 => AuthError.invalidCredentials,
+        _ when status == 429 => AuthError.factorCooldown,
+        _ => AuthError.unavailable,
+      });
+  static FactorException factorFailure(int status, BackendDTO dto) =>
+      FactorException(switch (dto.error) {
+        'expired' => FactorError.expired,
+        'locked' => FactorError.locked,
+        'invalid' => FactorError.invalid,
+        _ when status == 429 => FactorError.cooldown,
+        _ when status == 401 => FactorError.expired,
+        _ => FactorError.unavailable,
+      });
+}