feat[design]: ✨ add gray background to horizontal navigation item

[coraliefeil]

Description

This tasks describes the Figma implementation for: #1377

The navigation item when used as header navigation / as a trigger that opens a dropdown / mega menu should display a gray background when active / dropdown is open.

Open Questions towards development

• [ ] Do we need an "active" attibute for this? @smfonseca

DoR

• [ ] Item has been estimated by the team
• [ ] Item dependencies have been identified and documented

DoD

• [ ] Brand approval (if neccessary)
• [ ] Figma documentation file incl. hand-off has been created/updated (if applicable)
• [ ] Tokens have been applied/updated (if applicable)
• [ ] Crosslinks are created/updated
• [ ] Implementation works successfully on figma branch