Can I do the stock-take on my phone standing in the cold room?

Yes. The dashboard works in a phone browser, so open Menu → Ingredients & suppliers and go to the Stock-take panel. The number already on the shelf is shown in grey beside each box, so you see the difference as you type. Leave anything you did not count blank and it is left alone, which means the cold room today and the dry store tomorrow is fine. Write a note such as Sunday count and press Save the count. The page tells you how many you counted and how many changed. A restaurant inventory management app is not something you install here, it is the same admin on a smaller screen.