Editor doesn't know and cannot save map files outside sandbox.

The map editor is actually thinking it is saving the map files in the correct `$HOME/.teeworlds/maps` folder:


This is actually not true. The editor saves the file inside the `/home/miguel/.var/app/tw.ddnet.ddnet/.teeworlds/maps` folder. And we cannot navigate out of that folder.

Reproduce the editor's save behavior and compare the displayed $HOME/.teeworlds/maps path with the actual /home/miguel/.var/app/tw.ddnet.ddnet/.teeworlds/maps location. Inspect the Flatpak sandbox configuration and confirm what change is needed so maps can be saved or reached outside the sandbox; done means the editor uses the expected path and navigation works.

Written by the indexing model from the issue text.