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

manifest.chrome.json

@@ -0,0 +1,36 @@
+{
+  "manifest_version": 3,
+  "name": "Proxy Switcher Fork By WT",
+  "version": "1.0.1",
+  "description": "Selectively apply proxy servers to specific domains.",
+  "minimum_chrome_version": "99",
+  "icons": {
+    "16": "icons/proxy-switcher-16.png",
+    "32": "icons/proxy-switcher-32.png",
+    "48": "icons/proxy-switcher-48.png",
+    "64": "icons/proxy-switcher-64.png",
+    "96": "icons/proxy-switcher-96.png",
+    "128": "icons/proxy-switcher-128.png",
+    "512": "icons/proxy-switcher-512.png"
+  },
+  "permissions": [
+    "proxy",
+    "storage",
+    "tabs",
+    "activeTab",
+    "webRequest",
+    "webRequestAuthProvider"
+  ],
+  "host_permissions": ["<all_urls>"],
+  "background": {
+    "service_worker": "background-service-worker.js"
+  },
+  "action": {
+    "default_popup": "popup.html",
+    "default_icon": "icons/proxy-switcher-16.png"
+  },
+  "options_ui": {
+    "page": "options.html",
+    "open_in_tab": true
+  }
+}