🛡️ Sentinel: Add Content Security Policy (CSP) header

[seonghobae]

🚨 Severity: MEDIUM (Security Enhancement)
💡 Vulnerability: Missing Content Security Policy (CSP) header, which increases the risk of Cross-Site Scripting (XSS) and other code injection attacks.
🎯 Impact: Without a CSP, the browser might execute malicious scripts if an attacker finds a way to inject them.
🔧 Fix: Added a <meta http-equiv="Content-Security-Policy"> tag to restrict the loading of external scripts and limit script/style sources to 'self' (and unsafe-inline for styles).
✅ Verification: Ensure the app loads without CSP errors in the console and test suite passes. Tests have been verified (npm run test:e2e).

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PR created automatically by Jules for task 16908347069119835868 started by @seonghobae

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• .jules/sentinel.md
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