feat(router): add multi-select search page route

- Add new route for reusable multi-select search page
- Update Routes class with new multiSelectSearchName constant
- Implement builder function for multi-select search page
- Allow customization of title, items, and initial selections
- Add support for custom item builder function

Author: fulleni

lib/router/router.dart

@@ -50,6 +50,7 @@ import 'package:flutter_news_app_mobile_client_full_source_code/settings/view/no
 import 'package:flutter_news_app_mobile_client_full_source_code/settings/view/settings_page.dart';
 import 'package:flutter_news_app_mobile_client_full_source_code/settings/view/theme_settings_page.dart';
 import 'package:flutter_news_app_mobile_client_full_source_code/shared/services/feed_decorator_service.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/shared/widgets/multi_select_search_page.dart';
 import 'package:go_router/go_router.dart';
 import 'package:logging/logging.dart';
 
@@ -339,6 +340,35 @@ GoRouter createRouter({
           );
         },
       ),
+      // --- Reusable Multi-Select Search Page Route (Top Level) ---
+      // This route provides a generic UI for selecting multiple items from a
+      // searchable list. It is used by other pages (like the source filter)
+      // to offload the complexity of list selection.
+      GoRoute(
+        path: '/multi-select-search',
+        name: Routes.multiSelectSearchName,
+        builder: (context, state) {
+          final extra = state.extra as Map<String, dynamic>? ?? {};
+          final title = extra['title'] as String? ?? 'Select';
+          final allItems = extra['allItems'] as List<dynamic>? ?? [];
+          final initialSelectedItems =
+              extra['initialSelectedItems'] as Set<dynamic>? ?? {};
+          // The itemBuilder is passed as a function to display the item name.
+          final itemBuilder = extra['itemBuilder'] as String Function(
+            dynamic item,
+          )? ??
+              (item) => item.toString();
+
+          // Since this is a generic page, we pass the dynamic types directly.
+          // The calling page is responsible for casting the result.
+          return MultiSelectSearchPage<dynamic>(
+            title: title,
+            allItems: allItems,
+            initialSelectedItems: initialSelectedItems,
+            itemBuilder: itemBuilder,
+          );
+        },
+      ),
       // --- Main App Shell ---
       StatefulShellRoute.indexedStack(
         builder: (context, state, navigationShell) {

lib/router/routes.dart

@@ -27,6 +27,7 @@ abstract final class Routes {
   static const entityDetailsName = 'entityDetails';
   static const globalArticleDetails = '/article/:id';
   static const globalArticleDetailsName = 'globalArticleDetails';
+  static const multiSelectSearchName = 'multiSelectSearch';
 
   // --- Feed Sub-Routes ---
   static const articleDetailsName = 'articleDetails';