refactor(headlines-search): rename Category to Topic

- Update widget name from CategoryItemWidget to TopicItemWidget
- Replace Category with Topic in variable names and routing
- Adjust UI elements to reflect the new naming convention

Author: fulleni

lib/headlines-search/widgets/category_item_widget.dart

@@ -4,27 +4,27 @@ import 'package:ht_main/entity_details/view/entity_details_page.dart';
 import 'package:ht_main/router/routes.dart';
 import 'package:ht_shared/ht_shared.dart';
 
-/// A simple widget to display a Category search result.
-class CategoryItemWidget extends StatelessWidget {
-  const CategoryItemWidget({required this.category, super.key});
+/// A simple widget to display a Topic search result.
+class TopicItemWidget extends StatelessWidget {
+  const TopicItemWidget({required this.topic, super.key});
 
-  final Category category;
+  final Topic topic;
 
   @override
   Widget build(BuildContext context) {
     return ListTile(
-      title: Text(category.name),
-      subtitle: category.description != null
+      title: Text(topic.name),
+      subtitle: topic.description != null
           ? Text(
-              category.description!,
+              topic.description!,
               maxLines: 2,
               overflow: TextOverflow.ellipsis,
             )
           : null,
       onTap: () {
         context.push(
-          Routes.categoryDetails,
-          extra: EntityDetailsPageArguments(entity: category),
+          Routes.topicDetails,
+          extra: EntityDetailsPageArguments(entity: topic),
         );
       },
     );