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/router.tsx

@@ -25,11 +25,13 @@ import SkillsPage from '@/app/home/skills/page';
 import ErrorPage from '@/components/ErrorPage';
 import BackendUnavailablePage from '@/components/BackendUnavailablePage';
 import PluginPagesPage from '@/app/home/plugin-pages/page';
+import RootLayout from '@/app/RootLayout';
 
 const Loading = () => <div>Loading...</div>;
 
 export const router = createBrowserRouter([
   {
+    element: <RootLayout />,
     errorElement: <ErrorPage />,
     children: [
       {