feat(category): add autocomplete for subcategories

lib/presentation/providers/restaurant_provider.dart

@@ -78,6 +78,21 @@ final categoriesProvider = StreamProvider<List<String>>((ref) {
   });
 });
 
+/// 세부 카테고리 목록 Provider
+final subCategoriesProvider = StreamProvider<List<String>>((ref) {
+  final restaurantsStream = ref.watch(restaurantListProvider.stream);
+  return restaurantsStream.map((restaurants) {
+    final subCategories =
+        restaurants
+            .map((r) => r.subCategory)
+            .where((s) => s.isNotEmpty)
+            .toSet()
+            .toList()
+          ..sort();
+    return subCategories;
+  });
+});
+
 /// 맛집 관리 StateNotifier
 class RestaurantNotifier extends StateNotifier<AsyncValue<void>> {
   final RestaurantRepository _repository;