Set up playwright UI testing

The main things to test are:

• When going through the screens. When the randomisers stop moving, take note of what they are and make sure they are the same as the current states (and states at the end).

• Make sure when changing from quarter notes to eighth notes (etc), the change in the sound is instant.

• Make sure stop / restart work as expected

• Make sure chosen tempo is in the right range.

• Make sure pattern string matches beat start point (if can access this)