feat: useBooks hook with TanStack Query and App integration

[erikhfj]

Data hook + the one place everything gets wired together.

Tasks

• src/hooks/useBooks.ts: useQuery keyed ['books', subject] calling searchBooksBySubject (from feat: Add TypeScript types and OpenLibrary API client #4); expose { books, isLoading, isError }
• App integration: temporary hardcoded subject (e.g. 'fantasy', marked with a TODO for M3), current-index state in App, loading/error/empty states in plain accessible HTML
• Wire BookCard (feat: BookCard component (cover, title, authors, year, subjects) #11), NavigationControls (feat: NavigationControls (prev/next with progress indicator) #13), BookJumpList (feat: BookJumpList (jump to a specific book) #14) — the wiring lands after those merge to avoid App.tsx conflicts; the hook itself is parallel-safe
• Tests with msw: renders first book, navigation changes displayed book, jump selection works, and fetch count stays 1 while navigating (no unnecessary API calls — graded criterion)

Acceptance criteria

• npm run lint / npm test / npm run build pass
• No manual useEffect fetching — TanStack Query only

Part of M2 — Core app (due Tue 8/9). Branch feat/<n>-use-books from main after PR #3 merges.