[Good first issue]: Add npm run test:unit script for existing tests

[FrankezeCode]

Summary

Unit tests exist in test/ but there is no npm script to run them. Add a simple script and document it so contributors can verify changes locally.

Acceptance criteria

• Add test:unit script to package.json that runs existing tests in test/
• Script exits 0 when all tests pass
• Document the command in README.md or CONTRIBUTING.md (one line is enough)
• Do not change test logic unless required for the script to work
• No unrelated dependency or lockfile changes

How to test

1. Run npm run test:unit
2. Confirm existing tests (CommunityId, MeetupSlug, FormatDate, etc.) all pass
3. Confirm docs mention the new command
4. Run npm run build — still passes