What changed

• Treats first_publish_year: 0 from the Open Library API as missing data
• Adds a test to make sure an invalid publication year is normalized to null

This prevents books with missing publication data from displaying First published: 0.

Manual testing

Tested with The Invisible Man by H. G. Wells, which previously displayed:

First published: 0

After the fix, the invalid publication year is no longer shown. The book is also placed last when sorting by both newest and oldest, as missing publication years are intended to be.

Valid publication years are still displayed normally.

Verification

• Tests pass - 71/71
• ESLint passes
• Prettier check passes
• Production build passes
• Manually verified in the browser

Closes #47