Price-adding improvements

[monsieurtanuki]

Some improvements are needed so that the "add price" experience gets smoother:

• feature: when we add a product, focusing on its "amount" text field
• feature: not displaying the "add product" button when the product list only has the empty product
• feature: not accepting a barcode that is already in the current product list
• Add prices: displaying a "text recap" like an actual receipt, possibly with the total amount #5417
• feature: add a "remember my choice during this session" for the "server look-up?"dialog after each price product scan - good for online and offline contexts
• bug: when we remove a product, there's some confusion with the amounts (looks like a typical flutter key issue)
• Add prices: add a WillPopScope effect ("you've changed data: are you sure you want to exit?") #5418

Part of

• Replicate the 'Add Multiple products' view from the Open Prices web app #5203