GHSA-WHXW-24JC-CWMV Kirby: External Initialization of the Panel on reverse proxy setups with the `Forwarded` header

TL;DR This vulnerability affects Kirby sites that have no configured user accounts and are running on publicly accessible servers behind a reverse proxy that sets the Forwarded: for=..., X-Client-IP, or X-Real-IP request header. It was possible to install the Panel = create the first admin user i...