feat(web): dynamic document title per route

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.

web/src/app/RootLayout.tsx

@@ -0,0 +1,9 @@
+import { Outlet } from 'react-router-dom';
+import { useDocumentTitle } from '@/hooks/useDocumentTitle';
+
+// Top-level route layout: drives the dynamic document title from the active
+// route and renders the matched child route via <Outlet />.
+export default function RootLayout() {
+  useDocumentTitle();
+  return <Outlet />;
+}