Unit test frontend code

# Feature request

Cover frontend JavaScript with unit testing.

## Summary

While the Ruby backend core is fully covered by Minitest unit tests, the JavaScript frontend has no testing framework in place. This certainly encouraged past regressions and will continue to do so. It also omits a layer of expected behavior documentation.

Short of committing test framework packages to the repo, this would involve introducing a package.json file solely for dev dependencies. A dependency install step in dev setup instructions would need to be added.

(As an unrelated followup: with this in place, consider migrating vendor dependencies to this file.)