🛡️ Sentinel: [HIGH] Fix XSS vulnerability in exercise URLs

[longestmt]

🚨 Severity: HIGH
💡 Vulnerability: Cross-Site Scripting (XSS) vulnerability was found in src/pages/exercises.js where user-provided exercise properties (specifically mediaUrl) were injected directly into DOM innerHTML attributes without protocol validation, allowing javascript: URI-based execution. Other properties were also missing basic escapeHTML sanitization.
🎯 Impact: If a malicious user imports an exercise or synchronization pulls a corrupted entry with a javascript: URL, executing the media link could trigger arbitrary script execution within the context of the user's local instance.
🔧 Fix: Implemented protocol validation to ensure mediaUrl explicitly starts with http:// or https:// before applying escapeHTML. Additionally, wrapped ex.name, ex.muscleGroup, ex.equipment, ex.category, and ex.instructions in escapeHTML. Documented the javascript: URI bypass logic in the Sentinel journal.
✅ Verification: Code logic confirmed by visual review and node -c src/pages/exercises.js. Application builds correctly with pnpm build.

---

PR created automatically by Jules for task 17361104265065323669 started by @longestmt

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.