feat: add GitHub-only Chrome extension chat toggle

[liu971227-sys]

Summary

• add a Chrome Extension (Manifest V3) entrypoint
• inject a toggle button in the lower-right corner on github.com/*
• show/hide the existing chat UI in an iframe panel
• switch one insecure script URL to HTTPS for extension compatibility

What this implements for issue #1

• extension approach using iframe (minimal integration path)
• toggle button to show/hide chat
• active only on GitHub domain via content script match
• basic setup docs in README

Closes #1

[gogl92]

Hi, thank you for the PR I got this error while testing it

Loading the script 'https://www.gstatic.com/firebasejs/8.5.0/firebase-app.js' violates the following Content Security Policy directive: "script-src 'self'". Note that 'script-src-elem' was not explicitly set, so 'script-src' is used as a fallback. The action has been blocked.

[gogl92]

Please take a look at the PR comment.