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.

backend/src/products/products.security.spec.ts

@@ -10,6 +10,7 @@ describe('Products controller security', () => {
     createPending: jest.fn(),
     updateCanonicalNamePrivate: jest.fn(),
     mergePrivate: jest.fn(),
+    updateCategoryMine: jest.fn(),
   };
 
   const aiServiceMock = {
@@ -107,6 +108,14 @@ describe('Products controller security', () => {
     expect(productsServiceMock.mergePrivate).toHaveBeenCalledWith(42, 10, 20);
   });
 
+  it('updateCategoryMine vidarebefordrar req.user.id', () => {
+    productsServiceMock.updateCategoryMine.mockResolvedValue({ id: 1, categoryId: 5 });
+
+    controller.updateCategoryMine(1, { categoryId: 5 } as any, { user: { id: 42 } } as any);
+
+    expect(productsServiceMock.updateCategoryMine).toHaveBeenCalledWith(42, 1, 5);
+  });
+
   it('suggestCategory använder canonicalName fallback name', async () => {
     productsServiceMock.findOne.mockResolvedValue({ id: 1, name: 'Mjolk', canonicalName: 'Mjolk 1L' });
     categoriesServiceMock.findFlattened.mockResolvedValue([{ id: 1, name: 'Mejeri' }]);