Resolve "Static Loading Screen PWA"

So this does actually have some animation now, but it's not the final one yet, that will need a bit more time unfortunately.

ToDos

• Functionality is done
• probably some refactoring
• Find CD Jasmine error
• Add documentation
• Add Unit tests
• adjust accessibility modifier after writing unit tests
• adjust E2E tests
• E2E test run: https://git.recoverycat.de/recoverycat/rcc-client/-/pipelines/14211
• test theming after deployment

Update Sat 10.8.24

when checking this morning, i noticed the splashscreen not working on safari and thus on all iOS devices.. So I had to figure out why and change the mechanism which I luckily did but then on top the DOMContentLoaded event listeners doesn't work on safari as well, so had to find a workaround for that as well. Should now be good. 🤞

Closes #1215 (closed)