refactor(account): improve ad trigger and navigation flow

- Convert onHeadlineTap to an async function
- Add await for interstitial ad presentation
- Check if context is still mounted before navigation
- Add await for navigation after ad is closed

Author: fulleni

lib/account/view/saved_headlines_page.dart

@@ -72,9 +72,15 @@ class SavedHeadlinesPage extends StatelessWidget {
             );
           }
 
-          void onHeadlineTap(Headline headline) {
-            context.read<InterstitialAdManager>().onPotentialAdTrigger();
-            context.goNamed(
+          Future<void> onHeadlineTap(Headline headline) async {
+            // Await for the ad to be shown and dismissed.
+            await context.read<InterstitialAdManager>().onPotentialAdTrigger();
+
+            // Check if the widget is still in the tree before navigating.
+            if (!context.mounted) return;
+
+            // Proceed with navigation after the ad is closed.
+            await context.pushNamed(
               Routes.accountArticleDetailsName,
               pathParameters: {'id': headline.id},
               extra: headline,