fix(content): complete migration of content management page to use topic

Updates the `ContentManagementPage` to fully align with the migration
from `Category` to `Topic`.

- Changes the `TabBar` label from "Categories" to "Topics".
- Updates the "Add New Item" button logic to correctly handle the
  `ContentManagementTab.topics` case and navigate to the
  `createTopicName` route.

Author: fulleni

lib/content_management/view/content_management_page.dart

@@ -84,7 +84,7 @@ class _ContentManagementPageState extends State<ContentManagementPage>
                   isScrollable: true,
                   tabs: [
                     Tab(text: l10n.headlines),
-                    Tab(text: l10n.categories),
+                    Tab(text: l10n.topics),
                     Tab(text: l10n.sources),
                   ],
                 ),
@@ -103,8 +103,8 @@ class _ContentManagementPageState extends State<ContentManagementPage>
                 switch (currentTab) {
                   case ContentManagementTab.headlines:
                     context.goNamed(Routes.createHeadlineName);
-                  case ContentManagementTab.categories:
-                    context.goNamed(Routes.createCategoryName);
+                  case ContentManagementTab.topics:
+                    context.goNamed(Routes.createTopicName);
                   case ContentManagementTab.sources:
                     context.goNamed(Routes.createSourceName);
                 }