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/commands/session-snapshot.ts

@@ -1,7 +1,9 @@
+import { webExtApi as api } from '../browser-api';
+import type { Tab, TabGroup } from '../types';
 import { normalizeGroupColor, queryTabGroups } from '../core';
 import type { SessionTab, StoredSession } from '../types';
 
-export function buildSessionSnapshot(tabs: chrome.tabs.Tab[], groups: chrome.tabGroups.TabGroup[]): SessionTab[] {
+export function buildSessionSnapshot(tabs: Tab[], groups: TabGroup[]): SessionTab[] {
   const groupById = new Map(groups.map(group => [group.id, group]));
   return tabs
     .filter(tab => Boolean(tab.url || tab.pendingUrl))
@@ -27,7 +29,7 @@ export function buildSessionSnapshot(tabs: chrome.tabs.Tab[], groups: chrome.tab
  * its change-detection signature. Shared by session.save and the autosave path.
  */
 export async function captureCurrentSession(): Promise<{ session: StoredSession; signature: string; tabCount: number }> {
-  const tabs = await chrome.tabs.query({});
+  const tabs = await api.tabs.query({});
   const groups = await queryTabGroups({});
   const sessionTabs = buildSessionSnapshot(tabs, groups);
   const signature = sessionSignature(sessionTabs);