feat: add BookCard component

[robertky]

Summary

Closes #11.

• src/components/BookCard.tsx: props-driven presentation component with no fetching or app state
• src/components/BookCard.css: responsive plain CSS matching the existing app style
• Shows a cover image with descriptive alt text, or a text placeholder when a cover is missing
• Handles missing authors, publish year, and subjects without breaking
• src/components/BookCard.test.tsx: snapshots for full and minimal book data; verifies title, authors, cover, and placeholder behavior

Validation

• npm run format:check
• npm run lint
• npm test
• npm run build

[erikhfj]

Code review pass (automated, thorough): no bugs found. Verified BookCard against src/types.ts and coverUrl() in src/api/openLibrary.ts; lint / test (8) / format / build all verified locally.

Branch logic checked: firstPublishYear !== null matches number | null; coverUrl() → null correctly routes to the placeholder branch (no undefined src); empty authors → "Unknown author"; empty subjects → list omitted — all covered by tests.

Three minor, non-blocking notes:

1. Unbounded subject chips (UX, low): some real books return ~66 subjects (checked live API, e.g. The Picture of Dorian Gray) — BookCard.tsx renders every chip, so the card can get very tall. Consider slicing to the first 5–8.
2. Cover-file 404 edge case (low): OpenLibrary occasionally returns a cover_i whose cover file does not exist — coverUrl builds a URL for any non-null id, so the browser would show a broken image instead of the "No cover available" placeholder. An onError fallback on the img would cover this.
3. key={subject} (caveat): assumes subject strings are unique per book. No duplicates seen in live API samples, so likely fine — just flagging the assumption.

Nice work overall — the a11y and fallback handling are solid.