Fix XSS vulnerability in markdown renderer

Description

CONFIDENTIAL SECURITY ISSUE

Our markdown renderer component is vulnerable to XSS attacks through unsanitized HTML rendering.

Vulnerability Details

• Severity: HIGH
• Component: MarkdownRenderer component
• Issue: Uses dangerouslySetInnerHTML without sanitization
• Attack Vector: User-supplied markdown with embedded scripts

Reproduction

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

Remediation

• Install DOMPurify library
• Sanitize all HTML before rendering
• Add CSP headers
• Write security tests

cc @sabrina @bill