Impacted files:
* `app_en.arb`: message for a "change currency too?" dialog
* `country_selector.dart`: new method to change the currency at the same time as the country if relevant
* `product_query.dart`: init currency if null
* `pubspec.lock`: wtf
* `pubspec.yaml`: upgraded `openfoodfacts` to `3.8.0`
* `user_preferences_country_selector.dart`: asking to change the currency at the same time as the country only if confirmed or relevant
* `welcome_page.dart`: explicitly asking to change the currency at the same time as the country