🔒 [security fix] Mitigate XSS via Unescaped Set/Song Rendering

[julesklord]

🎯 What: Fixed multiple instances of unescaped data rendering in js/app.js.
⚠️ Risk: Potential for Cross-Site Scripting (XSS) if song metadata, set indices, or other dynamic values were maliciously crafted.
🛡️ Solution: Applied the existing esc() utility function to all interpolated variables in HTML template strings assigned to innerHTML.

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PR created automatically by Jules for task 4432856460468397756 started by @julesklord

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