Hi,
There is no such app as far as I know. However, it's a good idea indeed.
For now, there is no easy way to update the stock of one product via a web service in HikaShop.
Developing web services is something we want to do soon in HikaShop.
So it would start from here and then have an app to handle the scanning and contacting the website to update the stock.
However, it's possible to also do it without web services if you write a small system plugin to act as a small web service and interact with the app.