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/classes/CommandRegistry.ts

@@ -1,3 +1,4 @@
+import { webExtApi as api } from '../browser-api';
 import { CommandGroup } from './CommandGroup';
 import type { CommandContext, CommandEntry, CommandSpec } from './CommandGroup';
 import { NavigationCommands } from '../commands/navigation';
@@ -74,7 +75,7 @@ export class CommandRegistry {
 /**
  * Builds the registry and registers every command group. The SessionCommands
  * instance is returned alongside because index.ts wires its lifecycle methods
- * (chrome.tabs.onActivated → activateLazyTab) and NativeConnection references it
+ * (api.tabs.onActivated → activateLazyTab) and NativeConnection references it
  * for the clients.rename_profile reconnect side-effect.
  */
 export function assembleRegistry(ctx: CommandContext): { registry: CommandRegistry; session: SessionCommands } {