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.

extension/test/browser-api.test.ts

@@ -0,0 +1,43 @@
+// @ts-nocheck
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { webExtApi } from '../src/browser-api';
+
+test('browser-api uses Firefox browser.* before Chromium chrome.*', () => {
+  const originalChrome = globalThis.chrome;
+  const originalBrowser = globalThis.browser;
+  const firefoxApi = { runtime: { id: 'firefox-api' } };
+  const chromiumApi = { runtime: { id: 'chromium-api' } };
+
+  try {
+    globalThis.chrome = chromiumApi;
+    globalThis.browser = firefoxApi;
+
+    assert.equal(webExtApi.runtime, firefoxApi.runtime);
+  } finally {
+    if (originalChrome === undefined) delete globalThis.chrome;
+    else globalThis.chrome = originalChrome;
+
+    if (originalBrowser === undefined) delete globalThis.browser;
+    else globalThis.browser = originalBrowser;
+  }
+});
+
+test('browser-api falls back to chrome.* in Chromium', () => {
+  const originalChrome = globalThis.chrome;
+  const originalBrowser = globalThis.browser;
+  const chromiumApi = { runtime: { id: 'chromium-api' } };
+
+  try {
+    globalThis.chrome = chromiumApi;
+    delete globalThis.browser;
+
+    assert.equal(webExtApi.runtime, chromiumApi.runtime);
+  } finally {
+    if (originalChrome === undefined) delete globalThis.chrome;
+    else globalThis.chrome = originalChrome;
+
+    if (originalBrowser === undefined) delete globalThis.browser;
+    else globalThis.browser = originalBrowser;
+  }
+});