feat(extension): add Firefox WebExtension support

- Add a neutral WebExtension API adapter that uses Firefox browser.* or Chromium chrome.* without mutating globals.
- Switch extension runtime code to the adapter and add Firefox-specific typings for tabs, windows, tab groups, storage, scripting, and native messaging ports.
- Fix Firefox temporary add-on instructions to load the packaged manifest with background.scripts instead of the Chromium service worker manifest.
- Detect Firefox in clients.list via runtime.getBrowserInfo and keep Chromium user-agent fallback support.
- Make navigate.open wait briefly for Firefox to replace initial about:blank with the requested URL.
- Add JS coverage for API selection, clients.list browser detection, and Firefox navigate.open URL polling.
- Bump package and extension version to 0.15.2.

tsconfig.json

@@ -4,7 +4,7 @@
     "module": "ESNext",
     "moduleResolution": "Bundler",
     "lib": ["ES2022", "DOM"],
-    "types": ["chrome"],
+    "types": ["chrome", "firefox-webext-browser"],
     "allowJs": false,
     "strict": false,
     "noImplicitAny": true,