feat(shared): create generic MultiSelectSearchPage

Introduces a new reusable `MultiSelectSearchPage` widget. This generic, stateful widget provides a standard UI for selecting multiple items from a long, searchable list.

This component will be used to improve the user experience for filtering by country on the source filter page, replacing the non-scalable horizontal `ChoiceChip` list with a more conventional and user-friendly vertical list with search functionality.

Author: fulleni

lib/shared/widgets/multi_select_search_page.dart

@@ -0,0 +1,126 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_news_app_mobile_client_full_source_code/l10n/l10n.dart';
+import 'package:ui_kit/ui_kit.dart';
+
+/// {@template multi_select_search_page}
+/// A generic and reusable page for selecting multiple items from a searchable
+/// list.
+///
+/// This widget is designed to be pushed onto the navigation stack and return
+/// a `Set<T>` of the selected items when popped.
+/// {@endtemplate}
+class MultiSelectSearchPage<T> extends StatefulWidget {
+  /// {@macro multi_select_search_page}
+  const MultiSelectSearchPage({
+    required this.title,
+    required this.allItems,
+    required this.initialSelectedItems,
+    required this.itemBuilder,
+    super.key,
+  });
+
+  /// The title displayed in the `AppBar`.
+  final String title;
+
+  /// The complete list of items of type [T] to be displayed and filtered.
+  final List<T> allItems;
+
+  /// The initial set of selected items.
+  final Set<T> initialSelectedItems;
+
+  /// A function that returns the display string for an item of type [T].
+  final String Function(T item) itemBuilder;
+
+  @override
+  State<MultiSelectSearchPage<T>> createState() =>
+      _MultiSelectSearchPageState<T>();
+}
+
+class _MultiSelectSearchPageState<T> extends State<MultiSelectSearchPage<T>> {
+  late final Set<T> _selectedItems;
+  final _searchController = TextEditingController();
+  String _searchQuery = '';
+
+  @override
+  void initState() {
+    super.initState();
+    _selectedItems = Set<T>.from(widget.initialSelectedItems);
+    _searchController.addListener(() {
+      setState(() => _searchQuery = _searchController.text);
+    });
+  }
+
+  @override
+  void dispose() {
+    _searchController.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizationsX(context).l10n;
+    final theme = Theme.of(context);
+
+    final filteredItems = widget.allItems.where((item) {
+      final itemName = widget.itemBuilder(item).toLowerCase();
+      return itemName.contains(_searchQuery.toLowerCase());
+    }).toList();
+
+    return Scaffold(
+      appBar: AppBar(
+        title: Text(widget.title, style: theme.textTheme.titleLarge),
+        actions: [
+          IconButton(
+            icon: const Icon(Icons.check),
+            tooltip: l10n.headlinesFeedFilterApplyButton,
+            onPressed: () => Navigator.of(context).pop(_selectedItems),
+          ),
+        ],
+      ),
+      body: Column(
+        children: [
+          Padding(
+            padding: const EdgeInsets.all(AppSpacing.md),
+            child: TextFormField(
+              controller: _searchController,
+              decoration: InputDecoration(
+                hintText: l10n.searchHintTextGeneric,
+                prefixIcon: const Icon(Icons.search),
+                border: const OutlineInputBorder(
+                  borderRadius: BorderRadius.all(
+                    Radius.circular(AppSpacing.sm),
+                  ),
+                ),
+              ),
+            ),
+          ),
+          Expanded(
+            child: ListView.builder(
+              itemCount: filteredItems.length,
+              itemBuilder: (context, index) {
+                final item = filteredItems[index];
+                final isSelected = _selectedItems.contains(item);
+                return CheckboxListTile(
+                  title: Text(widget.itemBuilder(item)),
+                  value: isSelected,
+                  onChanged: (bool? value) {
+                    if (value != null) {
+                      setState(() {
+                        if (value) {
+                          _selectedItems.add(item);
+                        } else {
+                          _selectedItems.remove(item);
+                        }
+                      });
+                    }
+                  },
+                  controlAffinity: ListTileControlAffinity.leading,
+                );
+              },
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}