test: clean test warnings and fixtures

[audemed44]

Summary

• remove the backend FastAPI 422 deprecation warning by switching to the non-deprecated status constant
• add a shared frontend test router helper and update async test helpers so mount-time fetches settle before assertions
• fix stale frontend test fixtures and remaining hook dependency warnings in EditBookModal
• keep the frontend production build green by updating the ChapterList volume fixture to match the current type shape

Validation

• npx tsc --noEmit
• npx eslint src --ext js,jsx,ts,tsx --fix
• npm test (frontend)
• npm run test:backend
• npm run build (frontend)