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/src/core/throttle.ts

@@ -1,3 +1,4 @@
+import { webExtApi as api } from '../browser-api';
 // Large-operation throttling, performance profile, and job-progress helpers.
 import type { Job, JobProgressUpdate } from '../types';
 
@@ -16,7 +17,7 @@ function debugLargeOperation(message: string) {
 }
 
 export async function hasAudibleTabs() {
-  const audibleTabs = await chrome.tabs.query({ audible: true });
+  const audibleTabs = await api.tabs.query({ audible: true });
   return audibleTabs.some(tab => !(tab.mutedInfo && tab.mutedInfo.muted));
 }
 
@@ -36,14 +37,14 @@ export async function runLargeOperation<T>(name: string, fn: () => Promise<T>):
 }
 
 export async function getPerformanceProfile() {
-  const { performanceProfile } = await chrome.storage.local.get<{ performanceProfile?: string }>("performanceProfile");
+  const { performanceProfile } = await api.storage.local.get<{ performanceProfile?: string }>("performanceProfile");
   return performanceProfile || "auto";
 }
 
 export async function setPerformanceProfile(profile: string) {
   const allowed = new Set(["auto", "normal", "gentle", "ultra"]);
   const performanceProfile = allowed.has(profile) ? profile : "auto";
-  await chrome.storage.local.set({ performanceProfile });
+  await api.storage.local.set({ performanceProfile });
   return { performanceProfile };
 }