feat: add updateCategoryMine endpoint to manage product category updates

- Implemented a new PATCH endpoint in ProductsController to update the category of a user's product.
- Added corresponding service method in ProductsService to handle business logic and validation.
- Created UpdateCategoryMineDto for request validation.
- Enhanced error handling for forbidden actions and not found resources.
- Updated API error mapping in Flutter to handle specific forbidden messages.
- Modified ProductPickerField to allow product creation directly from the picker.
- Added tests for the new endpoint and service method to ensure proper functionality and error handling.

flutter/lib/core/ui/product_picker_field.dart

@@ -36,6 +36,9 @@ class ProductPickerField extends StatelessWidget {
   /// If set, the picker bottom sheet opens with this text pre-filled in the search field.
   final String? initialQuery;
 
+  /// Optional callback to create a new product directly from the picker.
+  final Future<ProductOption?> Function(String name)? onCreate;
+
   const ProductPickerField({
     super.key,
     required this.products,
@@ -46,6 +49,7 @@ class ProductPickerField extends StatelessWidget {
     this.label = 'Produkt',
     this.errorText,
     this.initialQuery,
+    this.onCreate,
   });
 
   @override
@@ -57,7 +61,7 @@ class ProductPickerField extends StatelessWidget {
               orElse: () => null,
             );
 
-    final interactive = enabled && !isLoading && products.isNotEmpty;
+    final interactive = enabled && !isLoading && (products.isNotEmpty || onCreate != null);
 
     return MouseRegion(
       cursor: interactive ? SystemMouseCursors.click : MouseCursor.defer,
@@ -81,7 +85,9 @@ class ProductPickerField extends StatelessWidget {
               : selected == null
                   ? Text(
                       products.isEmpty
-                          ? 'Inga produkter tillgängliga'
+                          ? (onCreate != null
+                              ? 'Inga produkter tillgängliga. Tryck för att välja eller skapa.'
+                              : 'Inga produkter tillgängliga')
                           : 'Tryck för att välja produkt',
                     )
                   : Text(selected.name),
@@ -98,6 +104,7 @@ class ProductPickerField extends StatelessWidget {
       value: value,
       label: label,
       initialQuery: initialQuery,
+      onCreate: onCreate,
     );
     if (!context.mounted) return;
     if (result == null) return;