🛡️ Sentinel: [improvement] Harden CSP and follow security best practices for event handling

[Shin5hi]

🚨 Severity: LOW/MEDIUM (Security Enhancement)
💡 Vulnerability: Missing restrictive CSP directives (object-src 'none', base-uri 'self') and use of legacy onclick property for event handling.
🎯 Impact: Reduces defense-in-depth against plugin-based XSS vectors and base-tag hijacking.
🔧 Fix:

• Added object-src 'none'; base-uri 'self'; to the CSP meta tag in index.html.
• Refactored the dynamically created close button in index.html to use addEventListener('click', ...) instead of onclick.
  ✅ Verification:
• Created and executed a Playwright script verify_security.py that confirmed the CSP update and the functional correctness of the close button.
• Verified visual state via frontend screenshots.
• Passed code review.

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PR created automatically by Jules for task 1543809379402847418 started by @Shin5hi

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