Loading UI is too slow / crashing on "big" ZIMs

[benoit74]

See e.g. https://library.kiwix.org/content/crashcourse_en_all_2024-07/index.html

Especially on first load, and more visible on "low-end" devices like a phone, the UI takes a lot of time to load.

What is concerning:

• there is no indication that work is ongoing, so it is confusing (I believed at first there is a bigger bug where the ZIM contains 0 video) => should we add a spinner until everything is ready?
• I believe (I do not have measures) that it is consuming lots of CPU (and maybe memory) since it is even way slower on CPU-limited devices like our users are using (it makes Chrome crash after about 2 minutes loading videos thumbnails in the UI on Orange tablet v2 for instance)

Issue is present on home page, but also on any video page since we load here the list of videos in the "playlist" component.