Improve test coverage

[muffix]

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

Test coverage is currently somewhat limited and focuses mostly on snapshot and additional manual testing. This makes it difficult to tell whether the app is working as expected.

Describe the solution you'd like

We should introduce further tests, especially around user interaction with the app. This will also make it easier to review external contributions.

Describe alternatives you've considered

N/A

Additional context