🔒 Restrict postMessage target origin

[pauloblikstein]

🎯 What: The overly permissive * target origin in postMessage calls within old site/tweaks-panel.jsx, index.html, and summer-studio/index.html was replaced with "/".

⚠️ Risk: By using the wildcard * as the target origin, any window or iframe could potentially intercept the messages sent via postMessage. This exposes sensitive state information and application settings to unauthorized domains, creating a significant security vulnerability (e.g. cross-site scripting risks and information disclosure).

🛡️ Solution: The target origin for all identified postMessage calls was changed from * to "/". The "/" origin string strictly restricts message delivery to recipients sharing the exact same origin as the sender. This eliminates the risk of message interception by untrusted third parties while maintaining the intended communication between trusted parent and child contexts.

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PR created automatically by Jules for task 1314283429518353823 started by @pauloblikstein

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