feat: refine digital library visual design

[robertky]

Summary

Closes #41.

Refines the existing Digital Library interface into a more deliberate reading-focused design without changing the application logic, API usage, or stored preferences.

Scope

• Adds a small shared CSS foundation for colors, typography, spacing, borders, focus states, and shadows.
• Refines the page header, Library/Favorites switcher, filters, sorting, navigation, and book-jump control into one consistent interface.
• Improves the book-card hierarchy: book title uses the display typeface, metadata remains readable, and long subject lists no longer crowd the action area.
• Makes the active favorite state visually distinct while preserving the existing favorite behavior.
• Presents the Favorites overview as a simple reading list rather than a collection of competing cards.
• Keeps book covers in the existing fixed frame and preserves the object-fit: contain behavior introduced in feat: consistent book covers #45, so covers are not cropped or stretched.

Files changed

• src/index.css — shared visual tokens and global page styling.
• src/App.css — page structure, header, responsive Library/Favorites switcher, controls, status messages, and footer.
• src/components/BookCard.css — book-card layout, metadata spacing, subject display, and narrow-screen presentation.
• src/components/FavoriteButton.css and FavoritesView.css — clear favorite states and the reading-list overview.
• src/components/SubjectFilter.css, SortSelect.css, BookJumpList.css, and NavigationControls.css — matching controls, hover states, disabled states, and visible keyboard focus.

User flows covered

• Browse books, filter by subject, sort, use Previous/Next, and jump directly to a book.
• Add or remove favorites and move between Library and Favorites without changing how favorites are stored.
• Use all controls with a keyboard; focused controls remain visibly identifiable.
• Use the interface on narrow screens, where Library and Favorites stack vertically and controls retain usable spacing.

Validation

• prettier --check on all changed CSS files
• eslint src
• vitest run — 70 tests passed
• vite build

Manual review requested

Please check the visual hierarchy and spacing on both desktop and a narrow viewport, especially long titles, ratings, long subject lists, and the active favorite state.

[rachelks]

Tested locally on both desktop and narrow screens, including long titles, subjects, favorites and the updated BookCard layout. Everything looks good and all checks pass.