🛡️ Sentinel: [security improvement] Harden Content-Security-Policy (CSP) against Object Injection and Mixed Content

[ImChong]

🛡️ Sentinel: [security improvement]

🚨 Severity: MEDIUM
💡 Vulnerability: The Content-Security-Policy (CSP) in index.html was missing the object-src 'none' and block-all-mixed-content directives.
🎯 Impact: Without object-src 'none', if an attacker finds an injection vulnerability, they could potentially execute XSS or other exploits via legacy plugin containers (like Flash or Java). Without block-all-mixed-content, active network attackers might inject unencrypted HTTP subresources on the HTTPS site.
🔧 Fix: Added object-src 'none'; block-all-mixed-content; to the CSP meta tag to explicitly disable objects/embeds and enforce secure asset fetching.
✅ Verification: Ran pnpm test and pnpm lint and confirmed no regressions were introduced. Also confirmed that the CSP string syntax is valid.

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PR created automatically by Jules for task 8833956263432158424 started by @ImChong

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