automated testing improvements

We already have some tests using Chrome/Puppeteer, but we should test more features and across Chrome and Firefox (and Safari, if possible).

https://vitest.dev/guide/browser.html

- Testing environments
- [ ] firefox
- [ ] service worker and updates between versions (begin by writing a wrapper script around docker to launch web server for given version)
- [ ] test video playback
- [ ] test offline behaviour

**Testing meta**
- [ ] replace all `await sleep` with `page.waitForX`
- [ ] mock API so that tests are not flaky (and can be extended for more than demo scenarios)