index.html.in: refactor the toggle_qr_focus() function

index.html.in: refactor the toggle_qr_focus() function

1. Adjust to the recent conversion of resize_ticket() to CSS
   transforms.
2. Refine the ticket dimensions to match those of CharmPass.
   This is still pretty hacky and may not work in all cases,
   but on a fresh page load in a desktop browser, everything
   is where it should be.
3. Simplify / clean up the code a little bit (using forEach,
   classList, etc.)