feat(extensions): manual verification help when browser launch fails

Expose a root navigator for global dialogs, show a fallback help sheet
with copy and reopen actions when verification URLs cannot launch, and
schedule the same prompt after a timeout during pending grants.

lib/services/app_navigation_service.dart

@@ -0,0 +1,8 @@
+import 'package:flutter/material.dart';
+
+class AppNavigationService {
+  static final GlobalKey<NavigatorState> rootNavigatorKey =
+      GlobalKey<NavigatorState>();
+
+  const AppNavigationService._();
+}