chore: set up regression test suite to support React upgrade

[bosun-ai]

Summary

Work in progress to upgrade React from 3.1.15 to 17.0.2. This PR documents initial setup for a regression test suite to support the upgrade. No significant application code changes are included yet.

Scope of changes

• Added regression test infrastructure:
  • regression-tests/jest.config.js for Jest configuration targeting regression tests.
  • regression-tests/setupTests.js to initialize RTL, MSW server, and jsdom environment.
  • regression-tests/button.regression.test.js with a passing test covering src/components/button behavior (renders children, disabled when isLoading, onClick not invoked).
• Added testing libraries and utilities in package.json:
  • @testing-library/react, @testing-library/jest-dom, @testing-library/user-event
  • msw and whatwg-fetch
• Added .nvmrc specifying Node 12 to ensure consistent local environment for running tests.
• Updated react-dom dependency reference to a semver version (preparatory step toward upgrading React).

Reasoning

Setting up a minimal, isolated regression test suite allows targeted verification of key components during the React version upgrade without impacting the existing test setup. The initial Button test provides a baseline for interactive behavior under loading/disabled states.

Regression test suite setup

• Directory: regression-tests/
• Tools: Jest, React Testing Library (RTL), MSW
• Included passing test for: src/components/button

Testing instructions

Run a single regression test using npm fallback:

• nvm use && npm run jest -- --config regression-tests/jest.config.js regression-tests/button.regression.test.js --runInBand

Impact and risk

• Low risk at this stage; changes are limited to test scaffolding and dependencies. Application runtime behavior is not modified yet.
• Ensures future React upgrade work has a test harness to catch regressions.

Follow-ups and open questions

• Complete React and react-dom upgrade to 17.0.2 and adjust related dependencies.
• Expand regression coverage to other critical components and flows.
• Align Node version and CI config with the regression test suite.
• Confirm compatibility of existing build tooling with React 17 (webpack/babel config, react-app-rewired).

This PR is a work in progress and is not intended to be merged.

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