Split initI18n() into separate server/client initialization paths
inspired by remix-i18next's architecture:
- initI18nServer(lng): no LanguageDetector, per-request language from
Accept-Language header
- initI18nClient(): LanguageDetector reads <html lang> first to match
server, then localStorage and navigator
- detectLanguage(request): parses Accept-Language for best match
Fixes rehydration errors caused by v26 removing initImmediate option
(replaced with initAsync) and LanguageDetector running on the server
where browser APIs don't exist.
Also makes <html lang> dynamic instead of hardcoded "en".
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
