🛡️ Sentinel: [security improvement] Harden CSP and secure event handlers

[Shin5hi]

🚨 Severity: MEDIUM (Security Enhancement)
💡 Vulnerability: Potential for accidental form submission and unauthorized form actions; reliance on legacy event handlers.
🎯 Impact: Minimal for the current static state, but provides defense-in-depth against future vulnerabilities like XSS or unauthorized data exfiltration.
🔧 Fix:

• Added form-action 'none' to Content Security Policy.
• Refactored dynamic button creation in JavaScript to use addEventListener and explicit type='button'.
• Updated the CSP SHA-256 hash in the meta tag to reflect the script changes.
• Added clarifying security comments (✅ Sentinel).
  ✅ Verification: Verified functionality and absence of CSP violations using a Playwright script (run locally). UI verification screenshot captured and confirmed.

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PR created automatically by Jules for task 2562263063332743938 started by @Shin5hi

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