fix(ui): enforce threshold selection for ranking rounds to prevent 500 error (#383)

[ayushshukla1807]

Fixes #383.

If a ranking round was submitted without a threshold, threshold: null was being sent to the API. Since the backend expects an int/float, this silently crashed the request with a 500 error.

I added a quick check in RoundNew.vue to trap this locally and trigger the standard form validation alert (alertService.error) instead, avoiding the network crash entirely.

[ayushshukla1807]

Just simulated this exact bug locally (vote_method='ranking' with a missing threshold).

With this patch, submitRound() actually stops the request and throws the normal alertService.error missing field alert, so the silent API 500 is completely avoided. Zero network calls made.

Ran the frontend build and basic tests too:

montage/tests/test_web_basic.py::test_home_client            PASSED   [ 50%]
montage/tests/test_web_basic.py::test_multiple_jurors        PASSED   [100%]

View local dev screenshots