fix(headlines-feed): mark AppUserFeedDecorator as completed when shown

- Add isCompleted: true flag to AppUserFeedDecoratorShown event
- This ensures that the feed decorator is marked as completed when it is shown to the user

Author: fulleni

lib/headlines-feed/bloc/headlines_feed_bloc.dart

@@ -374,6 +374,7 @@ class HeadlinesFeedBloc extends Bloc<HeadlinesFeedEvent, HeadlinesFeedState> {
       AppUserFeedDecoratorShown(
         userId: currentUser.id,
         feedDecoratorType: event.feedDecoratorType,
+        isCompleted: true,
       ),
     );
     // Remove the dismissed decorator from the current feedItems list to