fix(auth): correct resend response type and improve error messages

apps/lib/features/auth/data/models/auth_response.dart

@@ -54,3 +54,13 @@ class SignupStartResponse {
     );
   }
 }
+
+class SignupResendResponse {
+  final String message;
+
+  const SignupResendResponse({required this.message});
+
+  factory SignupResendResponse.fromJson(Map<String, dynamic> json) {
+    return SignupResendResponse(message: json['message'] as String);
+  }
+}