- Introduced a new `DownloadedBooksListView` to retrieve books downloaded by the current user.
- Updated `urls.py` to include the new endpoint `/books/downloaded/`.
- Removed the `is_downloaded` parameter from the `BookListView` to streamline the book listing process.
- Simplified the bookmark creation process by using request data directly in the serializer.
- Introduced a new `perform_create` method to save the user context during bookmark creation.
- Enhanced the book listing functionality to filter books by download status for the current user, using a new query parameter `is_downloaded`.