refactor(headline-details): use List.from instead of List.of

- Replace List.of with List.from for creating a shallow copy of savedHeadlines list
- This change improves code readability and follows the more common practice in the Flutter community

Author: fulleni

lib/headline-details/view/headline_details_page.dart

@@ -205,7 +205,7 @@ class _HeadlineDetailsPageState extends State<HeadlineDetailsPage> {
               .where((h) => h.id != headline.id)
               .toList();
         } else {
-          updatedSavedHeadlines = List.of(currentPreferences.savedHeadlines)
+          updatedSavedHeadlines = List.from(currentPreferences.savedHeadlines)
             ..add(headline);
         }