Add Chrome Extension (Manifest V3) for GitHub Chat

[andres170788]

Summary

Implements issue #1 — converts the existing web chat into a fully functional Chrome Extension (Manifest V3) that runs exclusively on github.com/* pages.

Features

• Toggle button in the lower-right corner to show/hide the chat panel (as requested in Add this to a Chrome Extension #1)
• iframe-based chat panel (320×480px) — minimal integration path as discussed
• Per-repo chat rooms — automatically detects owner/repo from the URL
• Firebase Firestore real-time messaging (reuses existing backend config)
• GitHub-themed dark UI that matches the platform aesthetic
• Toolbar icon click also toggles the chat
• XSS protection via HTML entity escaping
• Domain restricted to github.com/* only (via manifest content_scripts.matches)
• Delete own messages support

Architecture

chrome-extension/
├── manifest.json        # Manifest V3 config
├── background.js        # Service worker (toolbar icon handler)
├── content.js           # Injects toggle button + iframe on github.com
├── content.css          # Toggle button & panel styles
├── chat.html            # Chat UI (loaded inside iframe)
├── chat.js              # Firebase chat logic (no inline scripts)
├── css/
│   ├── chat.css         # Chat panel styles
│   └── normalize.css    # CSS reset
└── images/
    ├── icon16.png
    ├── icon48.png
    └── icon128.png

How to Install & Test

1. Clone this branch
2. Go to chrome://extensions → Enable Developer Mode
3. Click "Load unpacked" → Select the chrome-extension/ folder
4. Navigate to any github.com page
5. Click the green chat bubble in the lower-right corner

Screenshot

The extension adds a floating toggle button on GitHub pages. Clicking it opens a chat panel where users can communicate in real-time, with messages scoped to the current repository.

Closes #1