🛡️ Sentinel: [security improvement] Harden CSP and secure dynamic button types

[Shin5hi]

🛡️ Sentinel: [security improvement]

This PR introduces security enhancements to the 5ive UX sample by hardening the Content Security Policy (CSP) and improving the robustness of dynamic element creation.

🚨 Severity: MEDIUM (Security Enhancement)

💡 Vulnerability:

1. Overly Permissive CSP: The original CSP used default-src 'self', which allowed various resource types by default.
2. Implicit Button Types: The dynamically created close button relied on default browser behavior, which could lead to unintended form submissions if the DOM structure changed.

🎯 Impact:

• A stricter CSP reduces the attack surface for various injection attacks.
• Explicit button types ensure consistent and secure behavior across different browsers and layouts.

🔧 Fix:

• Updated the CSP meta tag to default-src 'none' and added form-action 'none'.
• Modified the notification script to set closeBtn.type = 'button'.
• Updated the script's SHA-256 hash in the CSP meta tag.
• Documented findings in .jules/sentinel.md.

✅ Verification:

• Verified using a Playwright script that triggers the notification, checks for functionality, and monitors the console for CSP violations.
• Confirmed that default-src 'none' does not break the current application as it only uses whitelisted styles and scripts.
• Verified that frame-ancestors was removed from the meta tag as it is not supported there.

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PR created automatically by Jules for task 15236986146194716561 started by @Shin5hi

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