feat: consistent book covers

[rachelks]

Summary

Improves the BookCard so covers are displayed consistently without being cropped or stretched. Covers now use a reserved area and keep their original aspect ratio, while missing or failed covers use the existing fallback.

The BookCard layout was also adjusted to stay more stable when navigating between books with different amounts of metadata and subjects.

Changes

• Added a consistent cover area for desktop and mobile
• Centered covers and preserved their aspect ratio with object-fit: contain
• Added fallback handling when a cover image fails to load
• Adjusted metadata spacing to reduce layout shifts between books
• Truncated long author lists visually while keeping the full text available
• Added a reserved subject area with scrolling when subjects are expanded
• Kept the subject expand/collapse control outside the scrollable area
• Improved spacing on narrow screens

Testing

• Added/updated BookCard tests for cover fallback, long authors and subject behavior
• Updated affected snapshots
• Manually tested different book covers and navigation between books
• Manually tested the layout on desktop and at 320 px width
• Manually verified subject expansion/collapse and favorites after merging the latest main
• All 66 tests pass
• npm run format:check passes
• npm run lint passes
• npm run build passes

AI assistance was used for discussing implementation options, resolving merge conflicts, and reviewing the changes. The implementation was tested and verified manually.

Closes #40

[robertky]

Looks good to me. The cover handling, fallback state, and stable BookCard layout are well covered by tests, and CI is green