Introduce tests and workflows to run them

[PAException]

https://docs.flutter.dev/testing/overview

Unit tests

• https://docs.flutter.dev/cookbook/testing/unit/introduction
• especially for storage and its components

Widget tests

• https://docs.flutter.dev/cookbook/testing/widget/introduction
• lower priority
• make sure custom widgets behave as expected

Integration tests

• https://docs.flutter.dev/cookbook/testing/integration/introduction
• high maintenance => only use for key features: substitutes (highly dependent)

other

mock http and api requests for integration testing (https://docs.flutter.dev/cookbook/testing/unit/mocking)

[PAException]

For CI/CD:
https://github.com/marketplace/actions/flutter-action