refactor(search): use generic keys for initial state

- Updated headline text key
- Updated subheadline text key

Author: fulleni

lib/headlines-search/view/headlines_search_page.dart

@@ -211,8 +211,8 @@ class _HeadlinesSearchViewState extends State<_HeadlinesSearchView> {
           return switch (state) {
             HeadlinesSearchInitial() => InitialStateWidget(
                 icon: Icons.search, // Changed icon
-                headline: l10n.headlinesSearchInitialHeadline,
-                subheadline: l10n.headlinesSearchInitialSubheadline,
+                headline: l10n.searchPageInitialHeadline, // Use new generic key
+                subheadline: l10n.searchPageInitialSubheadline, // Use new generic key
               ),
             // Use more generic loading text or existing keys
             HeadlinesSearchLoading() => InitialStateWidget(