Add fetchpriority attribute to main book cover image

[cdrini]

Small perf improvement to mark the main cover on a book page as fetchpriority="high", as suggested by pagespeed insights

Technical

Testing

Screenshot

Stakeholders

[Copilot]

Pull Request Overview

This PR adds the fetchpriority="high" attribute to the main book cover image on book pages as a performance optimization, following PageSpeed Insights recommendations.

• Adds fetchpriority="high" attribute to improve LCP (Largest Contentful Paint) metrics

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[RayBB]

I see the same lighthouse warning on master locally.
With your branch on my local env I see the fetchpriority="high" on the correct element:

Unfortunately, I still see the warning about it missing.

It's possible it's an issue with localhost?
Anyway, I'm approving because it seems to be the right thing to do in any case even if it doesn't fix the warning on localhost.

[cdrini]

Apparently "Important: When preloading the image, ensure fetchpriority=high is set on the preload as well as the image."

https://developer.chrome.com/docs/performance/insights/lcp-discovery

But it's still showing the same in my testing :/ I'm not certain if this is making much of a difference, since I see the request in a similar queueing both before / after :/ But it's best practice, I guess.