🛡️ Sentinel: [HIGH] Fix XSS vulnerability in markdown rendering

[khangnghiem]

🚨 Severity: HIGH
💡 Vulnerability: The Markdown rendering logic in site/app.js passed unsanitized output from marked.parse() directly into innerHTML.
🎯 Impact: Allowed execution of arbitrary JavaScript if a user crafted malicious markdown, leading to Cross-Site Scripting (XSS).
🔧 Fix: Implemented conditional DOMPurify sanitization. The application now sanitizes the markdown string before DOM insertion. If DOMPurify is unavailable, the fallback now safely escapes the raw markup instead of "failing open".
✅ Verification: Ran syntax checks on site/app.js, verified local tests cargo test and pnpm test, and validated code against repository security bounds.

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PR created automatically by Jules for task 9096514022059591812 started by @khangnghiem

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