Add spinner when waiting for ZIM to load viewer iframe content on kiwix-server

[benoit74]

When one loads a big ZIM with the iframe viewer, behavior is very disturbing. The viewer is pretty fast to load, but the iframe takes considerably more time. The fact that iframe content takes too much time to load can probably be optimized, but anyway there will be situations where the iframe will take time to load.

The problem is that while this iframe is loading, we have no visual indicator that something is going on. Since viewer has loaded, Firefox (in my case, don't know about other browsers) remove its own spinner indicating that page is loading.

We need to have our own visual indicator showing that something is still going on while iframe content is loading.

See video below for an example (I did not hit any button or anything else, load behavior is exactly as experienced by the users, I just showed network traffic at some point to indicate what was going on, but it misses the call which takes time to complete).

Screen.Recording.2025-02-13.at.10.07.21.mov