Migrate i18n from react-intl to next-intl, eliminating the formatjs
compilation pipeline. Translation files now live as nested JSON in
public/intl/messages/ (single source of truth), removing the duplicated
src/lang/ directory and the copy/compile build steps. The useMessages()
hook API is preserved so all 195+ consumer components are unchanged.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>