QR code support

[asdalo21]

Thank you for your project—it's fantastic, and I appreciate all the effort you've put into it.

I have a suggestion for an additional feature: when opening the website from a Kobo device, it would be very useful to display a QR code containing the URL along with the unique key.

Thanks again for your hard work!

[Fattur]

I opened a pr request for this:
#59