docs(frontend): use the real PageShell in the layout example (closes #196)

[antosubash]

Summary

Completes #196. The "full docs review & sync" (#205) replaced almost every nonexistent frontend hook/component reference, but left one in the Layouts JSX-composition example in docs/frontend/pages.md:

<AuthenticatedLayout>
  <PageHeader title="Orders" />   // PageHeader is not exported by @simple-module-py/ui
  ...
</AuthenticatedLayout>

PageHeader is one of the nonexistent components #196 is about — it isn't exported anywhere in packages/ui. This swaps it for the real PageShell page wrapper (and adds it to the import), matching the rest of the docs.

After this, a repo-wide grep for the names #196 enumerated (useAuth, useMenus, useFlash, PageHeader, DataTable, @simple-module-py/i18n-react) is clean.

Issues closed

• Closes [docs][frontend] Frontend docs reference hooks/components that don't exist (useAuth/useMenus/useFlash/PageHeader/DataTable, @simple-module-py/i18n-react) + wrong SharedProps.auth.user shape #196

Note: #197 and #198 were also fully addressed by #205 already — they can be closed directly (no diff needed); this PR only carries the one residual #205 missed.