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.)

该 issue 没有列出文件、测试或前端入口点。首先定位前端 JavaScript 和开发环境设置说明，然后确定所需的测试框架和包配置；完成的标准是能够通过文档化的开发环境运行前端单元测试。

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