added persisted app settings

lib/service/json_file_service.dart

@@ -14,21 +14,10 @@ class JsonFileService {
     required List<Bookmark> bookmarks,
   }) async {
     try {
-      final dir = await _directoryPath;
+      final dir = await selectDirectoryPath();
       if (dir.isEmpty) return false;
 
-      final data = {
-        'collections': collections.map((c) => c.toJson()).toList(),
-        'bookmarks': bookmarks.map((b) => b.toJson()).toList(),
-      };
-      final json = jsonEncode(data).codeUnits;
-      final file = XFile.fromData(
-        Uint8List.fromList(json),
-        mimeType: 'application/json',
-        name: constants.jsonFileName,
-      );
-
-      file.saveTo('$dir/${constants.jsonFileName}');
+      saveDataToFile(collections, bookmarks, dir);
     } catch (e) {
       return false;
     }
@@ -65,7 +54,31 @@ class JsonFileService {
     }
   }
 
-  static Future<String> get _directoryPath async {
+  static Future<bool> saveDataToFile(
+    List<Collection> collections,
+    List<Bookmark> bookmarks,
+    String directory,
+  ) async {
+    try {
+      final data = jsonEncode({
+        'collections': collections.map((c) => c.toJson()).toList(),
+        'bookmarks': bookmarks.map((b) => b.toJson()).toList(),
+      }).codeUnits;
+
+      final file = XFile.fromData(
+        Uint8List.fromList(data),
+        mimeType: 'application/json',
+        name: constants.jsonFileName,
+      );
+
+      file.saveTo('$directory/${constants.jsonFileName}');
+    } catch (e) {
+      return false;
+    }
+    return true;
+  }
+
+  static Future<String> selectDirectoryPath() async {
     if (Platform.isAndroid) {
       return await getDirectoryPath(
             initialDirectory: constants.defaultAndroidExportDirectory,