add: JavaScript SEO Checker - #9

Summary

• Adds one free JavaScript SEO Checker entry under the existing frontend category.
• The entry links directly to the public Mydentify tool route: https://mydentify.com/tools/javascript-seo-render-checker
• The description is based on the official tool page and states open_source: false accurately.

Checklist

• One item in this PR
• Lowercase, URL-safe slug matches the filename
• Accurate description and free pricing
• No reciprocal link, vote, review, or paid placement requested
• Self-submission disclosed: Mydentify is maintained by the submitter