Proper error handling for ranking

<img width="519" height="136" alt="Image" src="https://github.com/user-attachments/assets/1c083999-6682-449b-b0d0-0897d6e76f8f" />

If two candidates in ranked-choice voting have the same rank, the server rejects it. There should be some kind of frontend error essage (or frontend checking) instead of just spitting raw JSON at the user. 