fix: Update z-indices to avoid layouting issues

[MoritzWeber0]

Update the z-indices according to the following order (lowest to highest):

.o-interactive-map__controls (removed z-index, had no effect)
.o-dropdown__menu (removed z-index, should inherit from context)
.overlay--content
.o-aside
.overlay--search
#search (+ .pagefind-ui__form::before)
.o-header
.o-header__skip-link
.o-header__curtain
.o-header__nav--open nav
.a-snackbar

I took the line number and multiplied it by 2 to give some flexibility to add new indices in between in the future.

Resolves #432
Resolves #425
Resolves #406

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[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

[therobrob]

LGTM