feat: refine responsive book viewer and subject tags

[robertky]

Summary

Closes #33.

• Makes the subject and sort controls responsive: side by side when there is space, stacked on narrow screens
• Refines the book card so long titles and tags wrap instead of forcing horizontal overflow
• Shows at most five OpenLibrary subject tags and adds a +N more indicator for the remainder
• Keeps the existing book navigation and favorites behaviour unchanged

Implementation

• src/App.tsx: groups the subject and sort controls in a shared responsive layout container
• src/App.css: adds the two-column desktop layout, narrow-screen spacing, and mobile heading sizing
• src/components/BookCard.tsx: derives the five visible subjects and remaining-subject count from the book data
• src/components/BookCard.css: improves grid sizing, long-text wrapping, subject-tag layout, and the narrow-screen cover ratio
• src/components/BookCard.test.tsx: tests that a long list is limited and reports the hidden count
• src/__snapshots__/App.test.tsx.snap and src/components/__snapshots__/BookCard.test.tsx.snap: deliberately updated for the new layout wrapper and accessible subject-list name

User flow

1. On a larger screen, subject and sorting choices are presented together without taking unnecessary vertical space.
2. On a narrow screen, the choices stack and remain full-width and readable.
3. A book with many OpenLibrary subjects displays its first five tags plus a clear count such as +7 more.
4. Long titles and tags wrap inside the card rather than expanding the page horizontally.
5. Adding a favorite still immediately changes the button state and keeps the book in the Favorites overview.

Validation

• npm run format:check
• npm run lint
• npm test — 52 tests passed across 12 test files
• npm run build

Checklist

• Scope is limited to feat: make the library view responsive and limit long subject lists #33
• Existing interactions remain covered by tests
• Snapshots were reviewed and updated deliberately
• No UI library or inline static styling added
• Reviewer: check the layout at a 320 px-wide viewport before merge

[rachelks]

The responsive layout works well at 320px and I don't see any horizontal overflow. One thing I noticed is that the book information can be difficult to read when it is displayed directly on top of the cover, especially the author and publication year here. This could probably be an even bigger issue with darker/busier covers. Could we improve the contrast on the narrow layout?

Also a small styling detail: I think a little more spacing between the subject tags and the "Add to favorites" button would make the mobile layout easier to read.

[rachelks]

Looks good now! The text on the book card is much easier to read on small screens, and the spacing before the favorite button is better.