Upgrade app to Flutter/Dart3

I don't know the old app, so it's hard for me to tell if these changes introduce new issues or not, but at least I got the app to work with NC27 and Flutter/Dart3. Unfortunately, I couldn't really test the iOS version, but at least the debug build worked there.

There are still some issues to addressed for the color scheme, but I couldn't find any show stoppers so far, so I'm convinced that these changes are a viable improvement.