simplified code across the app and consolidated duplicated helpers

lib/view/pages/files/search/local_cache_search.dart

@@ -5,6 +5,7 @@ import 'package:flutter/foundation.dart';
 import '../../../../api/cache_store.dart';
 import '../../../../api/marianumcloud/webdav/queries/list_files/cacheable_file.dart';
 import '../../../../api/marianumcloud/webdav/queries/list_files/list_files_response.dart';
+import '../data/webdav_path.dart';
 
 /// Document key prefix used by `ListFilesCache._documentId`.
 const String _folderCachePrefix = 'wd-folder-';
@@ -42,18 +43,16 @@ List<CacheableFile> buildLocalCacheIndex(List<String> payloads) {
 /// Files in [index] whose name contains [query] (case-insensitive).
 ///
 /// [pathScope] restricts results to entries whose WebDAV path starts with
-/// the given folder. Pass an empty list (or null) to search globally.
+/// the given folder. Pass an empty list to search globally.
 List<CacheableFile> searchLocalCacheIndex(
   List<CacheableFile> index,
   String query, {
-  List<String>? pathScope,
+  List<String> pathScope = const [],
 }) {
   final trimmed = query.trim();
   if (trimmed.isEmpty) return const [];
   final needle = trimmed.toLowerCase();
-  final scopePrefix = pathScope == null || pathScope.isEmpty
-      ? ''
-      : '${pathScope.join('/')}/';
+  final scopePrefix = webdavFolderPath(pathScope);
   return [
     for (final file in index)
       if (file.lowerName.contains(needle) &&