Add: XSS Defender quest

[Akshatshukla-25]

What does this PR do?

Closes #819

Adds XSS Defender (quests/xss-defender.html) under the JavaScript Fundamentals category. The quest is an interactive web security game where players act as security engineers securing a web application against Cross-Site Scripting (XSS) attacks by configuring CSP headers (script-src 'self'), applying HTML entity encoding, and implementing DOM sanitization policies.

Type of Change

• New tool / quest
• Enhancement to existing tool
• Bug fix
• Documentation update
• Build system / infrastructure

Screenshot

Checklist

• I have read the Contributing Guide
• My branch follows the naming convention (add/tool-name, feat/description, or fix/description)
• I have tested my changes locally in at least one modern browser

For new tools / quests:

• The quest is a single self-contained .html file in the quests/ folder
• The filename is kebab-case (xss-defender.html)
• I added an entry to data/quests.json with all required fields
• I added a screenshot as quests/xss-defender.png next to the HTML file
• No JS frameworks or heavy UI libraries
• I ran node scripts/sort-norm.js data/quests.json to sort and normalize the quests registry
• I ran node scripts/build.js and verified the landing page renders correctly

[praveenscience]

👍