…found

Impacted files:
* `app_en.arb`: added 2 "add" and 4 "price product" labels
* `app_fr.arb`: added 2 "add" and 4 "price product" labels
* `edit_product_page.dart`: minor refactoring
* `get_prices_model.dart`: minor refactoring
* `price_amount_card.dart`: added the case of "no product yet"
* `price_amount_model.dart`: added "no product yet" check
* `price_meta_product.dart`: 2 more cases - "no product yet" and "not found product"
* `price_product_list_tile.dart`: minor refactoring
* `price_product_search_page.dart`: now we accept "not found" products and we don't force the server lookup
* `prices_card.dart`: minor refactoring
* `product_price_add_page.dart`: minor refactoring
* `user_preferences_account.dart`: added "Add receipt" and "Add price tags"