Temporary setup of alternative to advertisements

lib/app_navigator.dart

@@ -0,0 +1,14 @@
+import 'package:flutter/material.dart';
+
+/// A stable handle on the app's navigator, for the few places that need to
+/// put something on screen from outside a widget's own build method:
+///
+///  - [AppState.onAdWatched]'s "remove ads for a year" upsell, which fires
+///    from deep inside a background sync rather than from a widget.
+///  - `showAdPlaceholder`, called by [AdService], which owns no
+///    [BuildContext] of its own (real ads are native overlays that never
+///    needed one).
+///
+/// Lives here rather than in `main.dart` so services and widgets can reach
+/// it without importing the app's entry point.
+final navigatorKey = GlobalKey<NavigatorState>();