🛡️ Sentinel: [HIGH] Fix XSS vulnerability in exercises library page

[longestmt]

🚨 Severity: HIGH
💡 Vulnerability: Cross-Site Scripting (XSS) via unsanitized data injected into DOM innerHTML strings, and missing protocol sanitization on dynamic href properties.
🎯 Impact: Could allow execution of malicious scripts if untrusted exercise data is synced or imported, leading to potential data exfiltration or unauthorized actions in the user's context.
🔧 Fix: Wrapped all dynamic interpolation variables (ex.name, ex.muscleGroup, ex.equipment, ex.category, ex.instructions) with escapeHTML in src/pages/exercises.js and introduced a new sanitizeUrl function to ensure only http/https URLs are allowed in the mediaUrl link. Added security comments for developers.
✅ Verification: Build succeeds cleanly using pnpm build, no syntax errors verified with node -c.

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PR created automatically by Jules for task 8902295141334438836 started by @longestmt

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