feat: unite overlay implementations

[therobrob]

• moved overlay handling in separate js-file
• migrated mobileMenu overlay to the global one
• refactored closeOverlay handling

ℹ️ i’ve decided to close all elements when clicking on the overlay. there is another option, if we wanna handle it different.

Resolves #349

[github-actions]

Hi there! 👋

Thank you for your contribution to the FIP Guide! 🚀
We appreciate your effort in making rail staff travel information more accessible. 🚄

Checklist before merging:

• Added a description to the Pull Request
• Checked the License of new pictures (non-commercial use without attribution)
• Modified content in English
• Modified content in German
• Modified content in French

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[MoritzWeber0]

Thanks for the simplification of the code base. I noticed just one small regression, which we should address.

[MoritzWeber0]

The mobile menu overlay should also cover the logo and the search icon. While the search item issue can easily be fixed by adding a higher z-index, the header bar / logo issue is more complex. In the current DOM structure, we can't hide it via the z-index because the z-index is inherited to children (and can't be higher than the parent). Since the nav element is part of the header, it can't have a higher z-index. We would need to move it out.