[Good first issue]: Add unit tests for formatJoinDate

[FrankezeCode]

Summary

formatJoinDate formats member join dates on profile. Add edge-case tests to prevent regressions.

What should change?

• Extend test/FormatDate.ts with cases for:
• Valid ISO string
• Invalid date string (returns original input)
• Empty string
• ISO with timezone

Acceptance criteria

At least 4 test cases added,
Tests pass locally,
Tests follow existing test file style,

Checklist of done conditions:

• New assertions in test/FormatDate.ts
• Invalid input handled as documented
• Test command documented in PR if non-obvious
• No changes to production behavior unless bug found (if so, note in PR)
• How to test:

How to test

Run tests locally:
node --test test/FormatDate.ts