cleanup and added unit tests

hartmann-foto-documentation-frontend/lib/dto/customer_dto.dart

@@ -1,3 +1,4 @@
+import 'package:fotodocumentation/dto/picture_dto.dart';
 import 'package:fotodocumentation/utils/date_time_utils.dart';
 
 class CustomerListDto {
@@ -38,28 +39,4 @@ class CustomerDto {
   }
 }
 
-class PictureDto {
-  final int id;
-  final String? comment;
-  final String? category;
-  final String image;
-  final DateTime pictureDate;
-  final String? username;
-  final CustomerListDto customerListDto;
 
-  PictureDto(
-      {required this.id, required this.comment, required this.category, required this.image, required this.pictureDate, required this.username, required this.customerListDto});
-
-  /// Create from JSON response
-  factory PictureDto.fromJson(Map<String, dynamic> json) {
-    return PictureDto(
-      id: json['id'] as int,
-      comment: json['comment'] as String?,
-      category: json['category'] as String?,
-      image: json['image'] as String,
-      pictureDate: DateTimeUtils.toDateTime(json['pictureDate']) ?? DateTime.now(),
-      username: json['username'] as String?,
-      customerListDto: CustomerListDto.fromJson(json['customer']),
-    );
-  }
-}