🛡️ Sentinel: [security improvement] CSP에 Trusted Types 적용

[seonghobae]

🛡️ Sentinel: [security improvement]

* **취약점 (Vulnerability)**: 향후 `innerHTML`과 같은 위험한 DOM 싱크가 도입될 경우 DOM 기반 XSS에 노출될 위험이 존재합니다.
* **영향 (Impact)**: XSS 공격으로 인해 민감한 데이터 탈취나 원치 않는 동작 수행의 위험이 있습니다. 
* **수정 (Fix)**: 애플리케이션이 현재 `textContent`와 같은 안전한 DOM API만을 사용하고 있으므로, 외부 정책이나 별도의 새니타이저 없이 네이티브하게 `require-trusted-types-for 'script'` 지시문을 CSP에 추가하여 Trusted Types를 강제했습니다.
* **검증 (Verification)**: 브라우저 콘솔에서 `textContent` 할당이 차단되지 않고 정상 동작하며, 기존 테스트(`test_i18n.html`)가 모두 통과하는지 Playwright를 통해 검증했습니다.

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PR created automatically by Jules for task 13642618764277543542 started by @seonghobae

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