ci: run vitest test suite in CI (#1)

[ghzhost]

Summary

Fixes #1 — the repository ships vitest.config.ts, src/test/setup.ts, and 15+ test files, plus vitest / @testing-library/react / jsdom in devDependencies and a "test": "vitest run" script in package.json — but the CI workflow never invoked npm test, so the suite silently bit-rotted with no signal when it broke.

Change

Added a dedicated test job to .github/workflows/ci.yml that runs npm ci && npm test on every push and pull_request, parallel to the existing build job.

  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with: {node-version: 20}
      - run: npm ci && npm test

No changes to package.json, vitest.config.ts, or any source files — those were already correct per the issue description.

Closes #1