test: cover complete library user flow

[rachelks]

Summary

• Added an App interaction test covering subject change, sorting, book selection and favorites in one flow.
• Verifies that only the subject change triggers an additional OpenLibrary request.
• Existing tests already cover loading, error/no-results states and storage persistence/fallbacks.

Checklist

• npm run format:check
• npm run lint
• npm test — 52 passed
• npm run build

AI-assisted: AI was used to review the existing test coverage and assist with the new interaction test. The changes were reviewed and verified manually.

Closes #32

[robertky]

Looks good. The test covers the main user flow and confirms that sorting and favorites do not cause unnecessary API requests.