Is your feature request related to a problem? Please describe.

As a Nextcloud Mail maintainer I want CI to tell me if the app's basic functionality works

Describe the solution you'd like

Add an end to end test that logs in and opens the Mail app, then asserts that the setup screen shows

This proofs that

1. The app can be installed
2. The app is listed in the navigation
3. The frontend loads

Describe alternatives you've considered

No response

Additional context

The recent breakage at #11012.