Add Firefox support and cross-browser compatibility layer

This update introduces full Firefox support using a new manifest-firefox.json (Manifest V2), adds a webext-compat.js compatibility layer for unified API usage, and refactors all extension scripts (background.js, content.js, popup.js) to use the new webext API. The build and development process is automated with new Python scripts (build.py, dev.py), and GitHub Actions workflows are updated to support multi-platform build, test, and release. Platform detection and capability abstraction are added, and all relevant files and manifests are updated for cross-browser compatibility.

popup.html

@@ -576,6 +576,7 @@
       <div id="status" class="status" style="display: none"></div>
     </div>
 
+    <script src="webext-compat.js"></script>
     <script src="popup.js"></script>
   </body>
 </html>