feat: add separate favorites view with cross-subject overview

[robertky]

Summary

Closes #38.

Adds a separate Favorites view where saved books remain available together, even when they were added from different subjects or after the page has been reloaded.

Changes

• Adds Library and Favorites view controls in src/App.tsx, with clear selected and keyboard-focus states
• Stores the small amount of book information needed by the overview in t19.favorites, rather than only storing a book key
• Keeps existing saved keys compatible: a legacy favorite is retained and filled with details when its book is shown again
• Keeps corrupt or unavailable localStorage safe by falling back to an empty favorites list
• Replaces the old subject-specific favorites section with src/components/FavoritesView.tsx, which shows all saved books together and orders them by title
• Keeps saving, removing, subject filtering, sorting, navigation, and the existing OpenLibrary request flow working without additional API calls

User flow covered

1. Save a book from one subject.
2. Change subject and save another book.
3. Open Favorites to see both books in one overview.
4. Reload the page and return to Favorites; both books remain available.
5. Remove a saved book directly from the overview.

Tests

• Updates favorite persistence tests to verify the stored book details
• Covers legacy key-only favorites, corrupt stored data, and favorites from different subjects after reload
• Covers switching between Library and Favorites views
• Covers alphabetical ordering in the favorites overview
• Deliberately updates the App snapshot for the new view controls and removal of the old inline favorites section

Validation

• npm run format:check
• npm run lint
• npm test — 62 tests passed
• npm run build

Checklist

• Favorites are collected independently of the active subject
• Favorite data remains local and does not create extra OpenLibrary requests
• Existing stored favorites remain usable
• The separate view works with keyboard focus and has an explicit selected state

[rachelks]

Looks good! One small responsive UI thing I noticed when testing: on narrow screens, I think the Library and Favorites buttons would work better stacked vertically instead of side by side