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/api/api_error_mapper.dart

@@ -4,6 +4,11 @@ import 'package:flutter/services.dart';
 import '../l10n/l10n.dart';
 import 'api_exception.dart';
 
+const _safeForbiddenMessages = {
+  'Du kan inte omkategorisera, ta bort eller ändra produkter som är globala.',
+  'Du kan bara omkategorisera dina egna produkter',
+};
+
 String mapErrorToUserMessage(Object error, BuildContext context) {
   final l10n = context.l10n;
   if (error is ApiException) {
@@ -11,7 +16,9 @@ String mapErrorToUserMessage(Object error, BuildContext context) {
       case ApiErrorType.unauthorized:
         return l10n.sessionExpiredError;
       case ApiErrorType.forbidden:
-        return l10n.forbiddenError;
+        return _safeForbiddenMessages.contains(error.message)
+            ? error.message
+            : l10n.forbiddenError;
       case ApiErrorType.server:
         return l10n.serverError;
       case ApiErrorType.network:

flutter/lib/core/api/api_paths.dart

@@ -13,6 +13,7 @@ class ProductApiPaths {
   static const deleted = '/products/deleted';
   static const merge = '/products/merge';
   static const mergePrivate = '/products/private/merge';
+  static String updateMineCategory(int id) => '/products/mine/$id/category';
   static String mergePreview(int sourceProductId, int targetProductId) =>
       '/products/merge-preview?sourceProductId=$sourceProductId&targetProductId=$targetProductId';
   static String setStatus(int id) => '/products/$id/status';