Shelfmark Search per Pablo Ruiz

Adds a Search in Shelfmark button to book pages, so you can jump directly from discovery sites to your Shelfmark library search.

Features
- Adds a one-click Shelfmark search button on supported book pages.
- Supports both in-page button injection and popup-based search.

Supported Sites
- Goodreads (goodreads.com/book/show/...)
- Hardcover (hardcover.app/books/...)
- More sites coming soon!