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

[khangnghiem]

🚨 Severity: HIGH
💡 Vulnerability: The output of marked.parse for rendering markdown notes in site/app.js was directly interpolated into innerHTML without sanitization, leaving the application vulnerable to XSS if a user could inject malicious HTML within the markdown content.
🎯 Impact: An attacker could potentially inject malicious scripts that would execute in the context of the user's session when viewing a malicious markdown note, leading to session hijacking or unauthorized actions.
🔧 Fix: Wrapped the marked.parse(processedNote) output with window.DOMPurify ? window.DOMPurify.sanitize(rendered, { ADD_ATTR: ['data-note-node', 'data-node'] }) : fallback to ensure the HTML is sanitized before assignment to innerHTML, whilst gracefully preserving the application's required custom attributes. The fallback uses native regex escaping.
✅ Verification: Ran cargo check, cargo test, cargo clippy, and cargo fmt to verify no regressions in the core workspace, and verified JS syntax with node --check.

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PR created automatically by Jules for task 17023163726940725422 started by @khangnghiem

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