Add tests for both Node.js and browser environments

## Summary

Currently, the lack of environment-specific tests allows build regressions to slip through — as seen in #41 (partially reverted in #45).

## Goal

Add tests (or CI jobs) that verify the package works correctly in **both** environments:

- **Node.js** — ensure the Node.js build is functional and not broken by changes.
- **Browser** — ensure the browser build is functional and not broken by changes.

## Motivation

PR #41 broke the Node.js build without being caught by existing CI. Having dedicated test runs for each target environment would catch such regressions early.

## Acceptance Criteria

- [ ] A test (or CI job) that exercises the Node.js build and fails if the build is broken.
- [ ] A test (or CI job) that exercises the browser build and fails if the build is broken.
- [ ] Both checks run on every PR.

## References

- PR introducing the regression: #41
- PR fixing the regression: #45

/cc @tomusdrw