🛡️ Sentinel: [HIGH] Fix XSS vulnerability via javascript: URI in href attributes

[longestmt]

🚨 Severity: HIGH
💡 Vulnerability: Potential XSS vulnerability through javascript: URI injection in user-controllable URLs assigned directly to href attributes (e.g., ex.mediaUrl and info.url).
🎯 Impact: An attacker could execute arbitrary JavaScript in the victim's browser if they click on a maliciously crafted link with a javascript: protocol.
🔧 Fix: Created a sanitizeUrl utility function that strictly enforces http:// or https:// protocols and escapes HTML. Applied this function to dynamic href injections in src/pages/exercises.js and src/pages/settings.js.
✅ Verification: Ran node -c to verify syntax and pnpm build to ensure the project compiles successfully.

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PR created automatically by Jules for task 14440949630535296284 started by @longestmt

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