feat(localization): add searching for subtitle localization

- Replace hardcoded subtitle with localized string
- Use l10n.searchingFor() to provide model type specific subtitle
- Improve internationalization support in headlines search page

Author: fulleni

lib/headlines-search/view/headlines_search_page.dart

@@ -247,8 +247,9 @@ class _HeadlinesSearchViewState extends State<_HeadlinesSearchView> {
               // Use LoadingStateWidget
               icon: Icons.search_outlined,
               headline: l10n.headlinesFeedLoadingHeadline,
-              subheadline:
-                  'Searching ${state.selectedModelType.displayName(context).toLowerCase()}...',
+              subheadline: l10n.searchingFor(
+                state.selectedModelType.displayName(context).toLowerCase(),
+              ),
             ),
             HeadlinesSearchSuccess(
               items: final items,