Step 3 (group 3 of 3) — completes the App Router migration. The Pages
Router is now fully gone (no more pages/ or _app.js).

- /cosmicfest: server metadata wrapper + CosmicFestClient ('use client')
  for the modal/keyboard state. Swapped the deprecated next/image
  layout="intrinsic"/objectFit props for modern width/height + style.
- Deleted the dead legacy shells components/ParVaguesHeader.js and
  components/ParVaguesFooter.js (only ever referenced by commented-out
  imports in CosmicFest).
- tests/jest.setup.js: mock next/navigation (App Router hooks) instead of
  next/router.
- CLAUDE.md: Pages Router -> App Router throughout (global CSS from
  app/layout.tsx, server-components-by-default + 'use client' leaves,
  generateStaticParams, the root-shell + ParVagues-sub-layout split,
  next/navigation in tests).

Full App Router build green; route-smoke 14/14.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>