Add QR code in demo app that links to demo in store

Often times when showing people the demo it can be helpful to quickly allow people to install the app themselves. The QR code to the demo app should therefore be embedded into the demo app.

Requested by Matthias.

The issue names no files or tests; start by locating the demo app entry point and identifying the store link and existing UI assets. Done means the demo app visibly includes a QR code that scans to the demo's store page.

Written by the indexing model from the issue text.