🛡️ Sentinel: Fix missing frame-src in CSP for GitHub Sponsors

[google-labs-jules]

This PR updates the Content Security Policy (CSP) in firebase.json to explicitly allow iframes from github.com. This is required for the 'GitHub Sponsors' card on the About Page, which was previously blocked by the default policy (or lack of frame-src). This aligns the security configuration with the application's functionality as verified by tests.

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PR created automatically by Jules for task 8662945522178517116 started by @albertoivo

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