GP: site split + navbar update

[MartinKolarik]

We're going to move the GP site under its own domain and treat it more as a separate site. The code base is going to remain shared for now. The separation is only about the front end.

This means:

• some changes in the URL structure (no /globalping prefix),
• navbar updates (see Figma).

I'm keeping this task only for stuff that can't be changed/deployed before the split. Any smaller updates that can be shipped right away get their own tasks and will be deployed even before the split.