Impacted files:
* `fetched_product.dart`: moved code from `ProductRefresher`
* `product_cards_helper.dart`: unrelated bug fix
* `product_dialog_helper.dart`: special case when the product was created locally but not uploaded/refreshed yet
* `product_field_editor.dart`: unrelated bug fix
* `product_list_item_simple.dart`: moved code to model
* `product_model.dart`: special case when the product was created locally but not uploaded/refreshed yet
* `product_refresher.dart`: moved code to `FetchedProduct`; minor refactoring
* `smooth_product_card_found.dart`: added a banner for product with pending changes.
* `up_to_date_product_provider.dart`: minor refactoring

Impacted file:
* `smooth_product_card_found.dart`: removed the "pending..." banner