Add lazy loading for covers in works merge tool

[RayBB]

The works merge page currently loads all book cover images upfront, even those that are not immediately visible in the viewport. This causes unnecessary network requests and slower page load times.

Describe the problem that you'd like solved

We need to implement lazy loading for book cover images on the works merge page to:

• Only load images when they come into the viewport
• Reduce initial page load time
• Decrease unnecessary network requests
• Improve overall performance

Screenshot showing multiple image requests on page load

Proposal & Constraints

There was a previous PR here that started this but wasn't finished.
If you'd like to work on this please review that PR and all the comments on it and incorporate the feedback.

To be assigned to his issue comment saying that you have read this PR and will incorporate that feedback into your own PR.

To test locally you'll need to add data by running something like
docker compose exec -e PYTHONPATH=. web bash -c "./scripts/copydocs.py /works/OL21868175W /works/OL41495W"

Additional context

Example merge page showing the issue:
https://openlibrary.org/works/merge?records=OL21868175W,OL41495W

Stakeholders

@cdrini

[Realmbird]

@RayBB recreated problem docker compose exec -e PYTHONPATH=. web bash -c "./scripts/copydocs.py /works/OL21868175W /works/OL41495W"

[zafazmii]

Hi @RayBB, can I take on this issue?

[RayBB]

@chongthepro please read the full issue and follow the instructions in it to be assigned.

[zafazmii]

Hi @RayBB, I have read the feedback in the previous PR and will incorporate it into mine.

[RayBB]

@chongthepro thanks! You're assigned. Good luck!