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

[khangnghiem]

🚨 Severity: HIGH
💡 Vulnerability: The output of marked.parse(processedNote) was directly appended to innerHTML without sanitization in site/app.js. This allows an attacker to inject arbitrary HTML and execute cross-site scripting (XSS) if the markdown is parsed from untrusted sources.
🎯 Impact: An attacker could potentially inject malicious scripts into the web application, leading to session hijacking, unauthorized actions on behalf of the user, or defacement of the page.
🔧 Fix: Implemented a conditional check to sanitize the rendered markdown using DOMPurify.sanitize() before injecting it into the DOM. This configuration ensures that standard HTML structure is maintained while stripping out potentially dangerous tags and attributes, but strictly retaining custom attributes data-note-node and data-node.
✅ Verification: Ran node --check site/app.js to ensure valid syntax. Also ran cargo check --workspace and cargo test --workspace to ensure no related tests failed. Evaluated code structurally to ensure fallback cases were untouched to prevent regression.

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PR created automatically by Jules for task 10330242291525855005 started by @khangnghiem

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