Improve usability on smaller display sizes

On some screens there seem to be some problems if the content exceeds the height of the viewport but doesn't become scrollable. (e.g. on the send to lightning address screen)

Also the QR code sizing needs some adjustments. (possibly also some max sizes)

Reproduce the issue on the send to lightning address screen using a small display and check how overflowing content and the QR code are laid out. Review the relevant mobile UI entry point, then verify that the content can scroll and that the QR code has appropriate sizing limits.

Written by the indexing model from the issue text.