fix(backend): return 400 instead of 500 when threshold is null in advance_round

[lgelauff]

Closes #383.

What changed

Frontend (cherry-picked from #384, authored by @Oyelakin-Mercy): validates that a threshold is selected before submitting the advance-round form, showing a specific error message (montage-required-threshold) rather than letting the request through.

Backend (admin_endpoints.py): the existing except KeyError did not catch the case where threshold is present in the request but set to null — float(None) raises TypeError, not KeyError, so the exception propagated as an unhandled 500. Fix catches TypeError and ValueError alongside KeyError.

i18n: adds the missing qqq.json documentation entry for montage-required-threshold.

Reproduction

python .claude/reproduce_383.py   # requires local instance with debug: true

Before: threshold: null → 500 Internal Server Error
After: threshold: null → 400 with "unset threshold. set the threshold and try again."