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

config-manager.js

@@ -140,7 +140,7 @@ class ConfigManager {
 // Create global instance of configuration manager
 const configManager = new ConfigManager();
 
-// Create global variable for use in other scripts
-window.ConfigManager = ConfigManager;
-window.configManager = configManager;
-window.DEFAULT_CONFIG = DEFAULT_CONFIG;
+// Expose globals in extension pages and service workers.
+globalThis.ConfigManager = ConfigManager;
+globalThis.configManager = configManager;
+globalThis.DEFAULT_CONFIG = DEFAULT_CONFIG;