[Good first issue]: Add unit tests for githubAvatarUrl helper

[FrankezeCode]

Summary

The contributor carousel uses githubAvatarUrl() from src/lib/early-contributors.ts. Add unit tests following the existing test/ pattern (CommunityId.ts, MeetupSlug.ts).

Acceptance criteria

• New test file e.g. test/EarlyContributors.ts
• Tests cover valid username → expected GitHub avatar URL format
• Tests cover custom size query param when provided
• Uses existing project test style (node:assert / node:test)
• All tests pass when run manually
• No production logic changes unless a bug is found and fixed

How to test

1. Run the new test file: npx tsx test/EarlyContributors.ts (or project test command if added)
2. Confirm all assertions pass
3. Intentionally break URL in test locally — confirm test fails
4. Revert and confirm green again

[abhiramvsmg]

Hi @FrankezeCode, I'd like to work on this — could you assign it to me?