🛡️ Sentinel: [HIGH] Fix mXSS vulnerabilities in manual SVG sanitization

[bobdivx]

🚨 Severity: HIGH
💡 Vulnerability: The manual SVG sanitizer apps/web/src/lib/sanitize-svg.ts used an insufficient blocklist. It allowed <foreignObject>, <use>, <animate>, <set>, <image>, and <math> which could lead to mutation XSS (mXSS) and other attack vectors when rendering untrusted Mermaid or Math blocks.
🎯 Impact: Attackers could inject arbitrary JavaScript by hiding it inside allowed elements, compromising user sessions and data.
🔧 Fix: Expanded the BLOCKED Set to include foreignobject, use, animate, set, image, and math tags to proactively mitigate these vectors.
✅ Verification: Ensure npm test passes in apps/web. Added test cases pass.

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PR created automatically by Jules for task 11682707313219212924 started by @bobdivx

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