Treat the xdns mask type as a multi-value setting and update forms accordingly. Inbound/outbound UdpMask models now return arrays for xdns (inbound: settings.domains, outbound: settings.resolvers) using Array.isArray checks. UI templates were split so 'header-dns' still uses a single domain string, while 'xdns' renders a tags-style <a-select> for multiple entries (domains/resolvers). Conditionals were made explicit (mask.type === ...) instead of using includes(). Changed files: web/assets/js/model/inbound.js, web/assets/js/model/outbound.js, web/html/form/outbound.html, web/html/form/stream/stream_finalmask.html.
