Executing another bash inside the shell hook seems to break running
commands with `nix-shell --run`, something we assumed to work in the CI
configuration.

It seems that the Qt wrapper is already set up at least in modern
Nixpkgs, so we can simply remove the code for spawning the wrapped bash.

We previously added support for using the kiosk without connman
installed, but it still crashed when started on systems without DBus
(i.e. any non-Linux platform).

With this change we add a stub implementation of the core proxy
abstraction, which is going to provide safe no-op responses on non-Linux
platforms, allowing to use the kiosk, albeit without proxy support.

To avoid pulling in dbus stuff on MacOS altogether.

This sets up a development environment for the kiosk on macOS, as the
default nix-shell is currently broken due to upstream issues with the
`pyqt6-webengine` package.

The environment is managed using virtualenv and the `poetry` package
manager instead.