Summary

Closes #21.

• Adds src/hooks/useFavorites.ts: stores unique book keys in localStorage under t19.favorites, restores them on load, and safely falls back to an empty list for invalid data
• Adds src/components/FavoriteButton.tsx: an accessible favorite toggle on the displayed book card, including aria-pressed and the book title in its accessible name
• Adds src/components/FavoritesView.tsx: a favorites section with each book’s title, author, and a separate remove control
• Updates src/components/BookCard.tsx and src/App.tsx to connect the toggle, saved keys, and favorites overview

User flow

1. Add the currently displayed book to favorites.
2. The book appears in the Favorites section.
3. Refresh or reopen the browser: the favorite remains saved.
4. Remove the book from the Favorites section or toggle it off from the book card.

Tests

• useFavorites tests storage updates, restoring saved keys, and corrupt localStorage data
• FavoriteButton tests accessible state and clicking the toggle
• FavoritesView tests title, author, and removal behavior
• App test covers adding, reloading, restoring, and removing a favorite
• Snapshots added deliberately for the new components and updated deliberately for App
• Tests use MSW only; no real network calls

Validation

• npm run format:check
• npm run lint
• npm test — 38 passed
• npm run build