🛡️ Sentinel: [HIGH] Fix XSS in markdown rendering

[khangnghiem]

🚨 Severity: HIGH
💡 Vulnerability: Missing HTML sanitization on the marked.parse output in site/app.js when rendering user-provided Markdown notes could allow execution of embedded malicious scripts.
🎯 Impact: A cross-site scripting (XSS) attack could execute arbitrary JavaScript in the context of the editor, potentially stealing session information or performing unauthorized actions.
🔧 Fix: Wrapped the marked.parse output with window.DOMPurify.sanitize(rendered, { ADD_ATTR: ['data-note-node', 'data-node'] }) if DOMPurify is available. Also preserved the existing fallback logic of using a safe, regex-escaped string if DOMPurify isn't loaded (preventing a fail-open scenario).
✅ Verification: Tested syntax with node --check site/app.js. Ran tests via cargo test --workspace and pnpm test inside fd-vscode. Journaled the critical learning in .jules/sentinel.md.

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PR created automatically by Jules for task 1499804241110270739 started by @khangnghiem

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