End to end tests with Playwright

[Ben Wiley]

After #44.

• We'll need to see what we can do for scenarios where we would want to validate that a sound is playing (maybe even a specific sound). We could possibly add some kind of function spy to validate that an audio buffer matching a stored optimized audio file or syro stream snapshot is sent to play, and use standard end-to-end testing techniques for the rest.
• One important visual element is the waveform but it's not simple to test. One thing we can do is take an element screenshot of the waveform element (not the whole page!) and perform a visual comparison.
• For uploading files we can do this using stored source file snapshots.
• For downloading files we can do this and compare against stored source file and optimized file snapshots.