The banner was fetching /translations-{locale}.json from the CDN as
static files, but translations are stored in the DB and served via
the public /api/v1/translations/{siteId}/{locale} endpoint.
Fixes:
- fetchTranslations() now calls the public API endpoint
- loadTranslations() takes (apiBase, siteId, locale)
- banner.ts passes apiBase and siteId to loadTranslations()
- i18n.test.ts updated to match new signature
