The browser tab title was hard-coded to 'LangBot' in index.html and never
changed. Add a useDocumentTitle hook that maps the active route to an
existing i18n key and sets document.title to '<page> · LangBot', driven by
a new top-level RootLayout route element. Re-runs on navigation and on
language change so the title stays localized. Falls back to the bare app
name for unmapped routes.
