[SECURITY] Fix XSS vulnerability in MarkdownRenderer

🔒 CRITICAL SECURITY FIX

This MR addresses a HIGH severity XSS vulnerability in the MarkdownRenderer component (Issue #9).

Vulnerability Details

• Component: MarkdownRenderer
• Severity: HIGH
• Attack Vector: XSS through unsanitized markdown content
• Affected Users: All users of ui-component-library

Exploit Example

<img src=x onerror="alert(document.cookie)">

Solution

✅ Added DOMPurify for HTML sanitization ✅ Configured strict whitelist of allowed HTML tags ✅ Removed all dangerous attributes (onerror, onclick, etc.) ✅ Added comprehensive security tests

Changes

• Added dompurify and isomorphic-dompurify packages
• Updated MarkdownRenderer to sanitize all HTML before rendering
• Added 4 security test cases to prevent regression

Testing

npm test -- MarkdownRenderer.test.tsx

All XSS attack vectors are now blocked.

Closes #9

cc @sabrina @bill