🔒 Sentinel: [HIGH] Fix XSS vulnerabilities in workout and history pages

[longestmt]

🚨 Severity: HIGH
💡 Vulnerability: Stored Cross-Site Scripting (XSS) via unsanitized user-controllable input (e.g., plan names, day names, exercise names, equipment strings) rendered directly into innerHTML strings in src/pages/workout.js and src/pages/history.js.
🎯 Impact: An attacker could execute arbitrary JavaScript within a victim's browser context by crafting a malicious plan, day, or custom exercise name and importing it via the application's data functionality. This breaks security boundaries and exposes IndexedDB data.
🔧 Fix: Wrapped all previously unescaped template variables in the affected UI functions with the centralized escapeHTML() utility from src/utils/sanitize.js.
✅ Verification: Ran pnpm build and Native Node Tests to confirm regressions weren't introduced and syntax is valid. Verified XSS mitigation logic behavior.

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PR created automatically by Jules for task 3442407995150504058 started by @longestmt

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