feat: add author and rating sorting for library books

[robertky]

Summary

Closes #36.

Adds two useful ways to order the already-loaded books and makes the rating data behind one of those choices visible on every relevant book card.

Changes

• Extends the existing OpenLibrary subject search in src/api/openLibrary.ts with ratings_average and ratings_count; it still makes one request per selected subject and fetches the same number of books
• Adds averageRating and ratingsCount to the normalized Book model in src/types.ts, with safe null fallbacks for missing or invalid values
• Adds Author A-Z and Highest rated to src/utils/sortBooks.ts and the existing sort select
• Orders authors by the first listed author and keeps books with no author after the books with author data
• Orders ratings from highest to lowest; books with the same rating use the number of ratings as a tie-breaker, and books without a rating stay visible at the end
• Shows Rating: X.X / 5 (N ratings) on each rated BookCard, independent of the selected sort option; books without a rating do not show an empty row

Tests

• Updates the MSW fixture books with rating data without introducing real network calls in tests
• Covers both new sort modes, equal ratings, and missing author/rating data in src/utils/sortBooks.test.ts
• Covers changing sort options without a second request and restoring Highest rated from sessionStorage in src/App.test.tsx
• Covers the visible rating and missing-rating state in src/components/BookCard.test.tsx
• Deliberately updates the affected App and BookCard snapshots

Validation

• npm run format:check
• npm run lint
• npm test — 56 tests passed
• npm run build

Checklist

• Author and rating sorting work entirely client-side
• Ratings are explained to the user rather than only affecting order invisibly
• Missing OpenLibrary fields are handled safely
• Existing filtering, navigation, favorites, sessionStorage, and no-extra-request behavior remain covered

[rachelks]

Re-reviewed after the update with main. Sorting by author and rating works as expected, and all checks pass. Looks good to me! 😊