Add detectLocale() that extracts the full locale tag (e.g. "de-DE")
from the request, not just the language. The journal root loader passes
it via LocaleContext so ClientDate renders with the correct locale on
both server and client — no more hardcoded "en-US" fallback.
Also fixes settings page hydration mismatch by using ClientDate for
passkey creation dates instead of inline toLocaleDateString().
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
