complete budget page

lib/clients/storage/json_storage_client.dart

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+import 'dart:io';
+
+import 'package:path_provider/path_provider.dart';
+
+class JsonStorageClient {
+
+  save(String filename, String data) async {
+    File file = await _getJson(filename);
+    await file.writeAsString(data);
+  }
+
+  Future<String> load(String filename) async {
+    File file = await _getJson(filename);
+    return file.readAsString();
+  }
+
+  delete(String filename) async {
+    File file = await _getJson(filename);
+    await file.delete();
+  }
+
+  Future<File> _getJson(String filename) async {
+    final dir = await getApplicationDocumentsDirectory();
+    final file = File('${dir.path}/$filename'); 
+    if (!file.existsSync()) {
+      file.createSync();
+    }
+    return file;
+  }
+}