Add Facebook username validation support (social category)

[ali-m07]

This PR adds Facebook username validation to the social category.

• Implements \ using the shared \ helper
• Uses the pattern https://www.facebook.com/ for profile URLs
• Treats HTTP 404 as available and HTTP 200 as taken

[kaifcodec]

@ali-m07 Thanks for the PR, great job, it works fine!

One suggestion: Facebook enforces strict username rules (allowed symbols, length, etc.). To make this validator more accurate, could you add a regex-based pre-check before calling status_validate()?

Suggested flow:

• If the username matches the Facebook pattern → call status_validate()
• If it doesn’t → return 2 with a clear message explaining what constraint failed

This avoids unnecessary requests and gives the user better feedback.
If you run into any issues with showing the validation error message, feel free to ask, and once updated, you can push the changes here and reopen the PR.