Add more tests.

Our reliability has been so low, adding more test would be beneficial. Things that can be tested:

probably good idea:

• e2e test for checkout
• e2e test for all navigation target
• e2e test for uploading photo
• e2e test for adding discount
• integration tests for repositories
• integration test for viewmodels

nice to have:

• e2e test for adding printer (requires real device and printer)
• unit tests for dao