Demo mode banner HTML, built on demand.

Deliberately a function, not a module-level constant: calling t() at module scope would construct the isolate's first IntlMessageFormat at import time, and that first construction loads ~13ms of ICU locale data. Since the layout imports this module on every page, a module-level t() here would pay that ICU init on every cold boot — before the request is even routed — for a banner that only ever shows in demo mode. As a function it runs only when the banner is actually rendered (and the layout already gates it on isDemoMode()), so requests that render nothing (webhooks, redirects, health checks) never pay it.