feat: show joined date on public contributor profiles

[atrip0305]

Summary

Adds a subtle "Joined X days/months ago" line to public contributor profiles using the profile 'created_at' timestamp. Recent joins are displayed relatively, while older joins are shown as an absolute month/year format.

Type of Change

• New feature
• UI / UX improvement

Related Issue

Closes #73

What was changed?

• Extended the profile query to fetch 'created_at'
• Added 'joinedAt' to the 'ProfileData' type
• Introduced a 'formatJoinDate()' helper for relative/absolute formatting
• Rendered a muted join date line below the contributor handle on public profiles

Checklist

• My code follows the project structure and conventions
• I tested this locally ('npm run dev')
• No hardcoded secrets or credentials

[vercel]

@atrip0305 is attempting to deploy a commit to the codersogs-3057's projects Team on Vercel.

A member of the Team first needs to authorize it.