fix: make sure loading widget hides when ReadiumReaderWidget is done

[AndreasLymalmMTM]

Description

Any loading widget passed to ReadiumReaderWidget stayed above the content after loading completed, often just less than a second, but in rare cases it stayed indefinitely.

The solution in this PR is to listen on the initial content being visually ready (with the new onReaderReady), instead of onPageChanged.

Before -> After

loading-widget-stay.mp4

loading-widget-hide.mp4

[ddfreiling]

Thanks @AndreasLymalmMTM. There seem to be an iOS integration test failing now. Can you run the integration tests locally and debug that?

[m-abs]

@AndreasLymalmMTM can you also merge main into this?

[AndreasLymalmMTM]

@ddfreiling @m-abs I'm on it!

[ddfreiling]

All iOS integration tests pass fine for this branch on my local machine, may be just a GH runner timing fluke

[AndreasLymalmMTM]

All iOS integration tests pass fine for this branch on my local machine, may be just a GH runner timing fluke

Yeah, that's what Copilot thought too when dianogsing the failing test. Let's see how it goes.

[ddfreiling]

These GH iOS runner are entirely unreliable. Merging this, thanks for catching this @AndreasLymalmMTM 🥇

[AndreasLymalmMTM]

These GH iOS runner are entirely unreliable. Merging this, thanks for catching this @AndreasLymalmMTM 🥇

Thanks!