add E2E test coverage

[koebel]

add basic E2E test coverage for the app

with the current implementation status, the following scenarios could be tested:

• preview 3D model
• toggle between standard and fullscreen mode
• rotate and zoom 3D model?
• reset model after modifying display position
• show prev/next model

TODO

• define scenario(s) (Scenarios for E2E tests #10)
• set up test environment (e2e test setup & test implementation #12)
• write tests for all scenarios (e2e test setup & test implementation #12)

[koebel]

it's challenging to test the steps in the scenario relating to rotate, zoom and reset 3D model, see comment
Since this is lower priority, there is a separate issue for this, see #15