Browser side
Front-end behavior
views/js/front.js binds to every form of the page containing a company field and a vat_number field (MutationObserver watches dynamically loaded forms). It mirrors the server rules: number required for a company (everywhere, or only in the EU depending on the setting), company required in a country reserved for businesses, official number formats. It shows an inline error and blocks submission — a browser-side safety net, server validation remaining the only reliable barrier.