Refactor QML to use StackView

The approach with the list of States continues being a mess. Reusing controls just makes it harder to share state with the C++ counterpart (can't even implement bidirectional sync for username). I also can't imagine how one would customize that to make a theme 😿

One idea is to define distinct screens for each state and manage these with StackView. The other ideas... I have none at the moment.