i18n switched to no_prefix, eliminating /undefined/ sitemap entries

The i18n config was generating sitemap URLs with a runtime token that resolved to the literal string undefined in production builds. The sitemap ended up containing entries like https://feedback.roaderly.com/undefined/project/board — invalid and confusing to crawlers.

The fix switches the i18n strategy to no_prefix (the default locale serves at the root, alternates only emit if real). With a single locale today, the prefix concept was creating more problems than it solved.

All /undefined/ entries are gone from the sitemap and the canonical URLs are now stable.