feat: add cross-browser builds and refresh project documentation

- add Chrome/Chromium Manifest V3 build with PAC proxy routing
- keep Firefox Manifest V2 and Android compatibility
- add Chromium API adapter and service worker
- add Chrome build script and Makefile target
- update README and add AGENT.md
- document installation, signing, permissions, themes, and platform
  limitations

options.html

@@ -243,6 +243,7 @@
         </footer>
     </div>
 
+    <script src="browser-api.js"></script>
     <script src="options.js"></script>
 </body>
 </html>