Resolve "Make QR scanner viewfinder transparent"

Closes #205 (closed)

Couldn't figure out how to do CSS background mixing, so instead I just reworked the boxes to fit around the QR scanner, which is transparent while the rest are 80% opacity black (as before). Easy to understand by this coloured version:

Downside is that the rounded corners don't fit perfectly. I think this is still an upgrade and in practice probably not noticeable, but if they are then we can always unround the green border edges. Looks a little worse, but ease of scanning is greatly improved.

I couldn't figure out how to make a local .apk to test yet, but I believe this should work. Kallisti if you can do a quick build and test before merging, that works too, or if not I'll figure it out at some point.