🛡️ Sentinel: [HIGH] Fix XSS vulnerability in href injection

[longestmt]

🚨 Severity: HIGH
💡 Vulnerability: A Cross-Site Scripting (XSS) vulnerability was found where dynamically generated href attributes (ex.mediaUrl in src/pages/exercises.js and info.url in src/pages/settings.js) could execute malicious JavaScript if a user supplied a javascript: protocol URI. escapeHTML alone does not protect against this as javascript: URIs do not necessarily contain HTML special characters.
🎯 Impact: An attacker could inject a malicious URL into the database (e.g., via sync or import), causing arbitrary JavaScript execution when a user clicks the "Watch Video" or "View Gist" links.
🔧 Fix: Added a sanitizeUrl function in src/utils/sanitize.js to explicitly validate that URLs start with http:// or https:// before escaping them. Applied this function to the relevant href injections in src/pages/exercises.js and src/pages/settings.js.
✅ Verification: Verified the application builds successfully (pnpm build) and unit tests pass (node --test). Added a journal entry detailing the learning and prevention strategy.

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PR created automatically by Jules for task 12402462297349079819 started by @longestmt

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