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

[khangnghiem]

🚨 Severity: HIGH
💡 Vulnerability: The output of marked.parse in site/app.js was directly interpolated into innerHTML without sanitization. If window.DOMPurify was unavailable, it failed open by rendering the unsanitized HTML.
🎯 Impact: An attacker could inject malicious scripts or HTML through user-provided Markdown content in notes or other rendered inputs, leading to Cross-Site Scripting (XSS).
🔧 Fix: Sanitized the output of marked.parse(processedNote) using window.DOMPurify.sanitize (allowing specific attributes like data-note-node). Implemented a secure fallback that regex-escapes HTML entities if DOMPurify is unavailable.
✅ Verification: Ran node --check site/app.js to confirm no syntax errors were introduced. Ran all tests via pnpm lint and pnpm test in the fd-vscode workspace to ensure no regressions. Reviewed the fallback implementation to confirm it does not fail open.

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PR created automatically by Jules for task 15003678215010760968 started by @khangnghiem

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