Vite >=4.3 rejects requests with a Host header not in the allowlist,
which broke status-page submissions and dev-server access for users
serving Checkmate behind a reverse proxy after upgrading.
Adds VITE_ALLOWED_HOSTS env var (comma-separated list). When unset,
falls back to allowing all hosts so existing self-hosted deployments
keep working without changes.
Closes#3397Closes#2951