better scannable "Add Second Device" QR code #4052
Description
there are several reports, that the QR code of the "Add Second Device" screen is hard hard to scan, esp. by some poor scanners.
the situation got worse by some iroh update, adding more data to the QR code (eg. add-second-device Qr code is more dense than most invite-links)
this issue is about improving the showing side, esp. when when the QR code is shown on a small, maybe dirty and dark screen
the currently used dc_backup_provider_get_qr_svg is anyway deprecated, so we should use dc_create_qr_svg(dc_backup_provider_get_qr()), which will shown more QR code when using the same svg width - also contrast to background is maximized. the borders are removed, but they're anyway no longer state-of-the-art nor good looking
the subtitle can probably be removed alltogether, it does not add much, context is pretty clear
before / possible after - and there can be even smaller devices ...
the same QR code layout is used for proxy already
in a subsequent core PR, we can consider to remove the overlay for DCBACKUP, which eats the error correction.
once this issue is closed, file issues for iOS and desktop