🛡️ Sentinel: [CRITICAL] Fix DOMPurify bypass in markdown parsing

[khangnghiem]

🚨 Severity: CRITICAL
💡 Vulnerability: site/app.js rendered user-provided markdown node notes to HTML via marked.parse without any DOM-level sanitization, making it directly vulnerable to Cross-Site Scripting (XSS) via innerHTML assignment.
🎯 Impact: Arbitrary script execution within the user's browser context if a malicious actor controls the markdown source notes or supplies malicious markdown syntax.
🔧 Fix: Add strict DOMPurify.sanitize conditioning for rendered HTML before assignment. If DOMPurify is unavailable, securely degrade to regex HTML text escaping inside <pre> tags. Added inline security comments.
✅ Verification: Ran syntax checks via node --check site/app.js and confirmed there are no build regressions by running cargo test --workspace.

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PR created automatically by Jules for task 10126509515156068490 started by @khangnghiem

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