Unit choice should be global

[johnny9]

• In the prototype, I set up an application-wide setting for the unit format. So if you toggle it in the form for one recipient, it also toggles it for all other recipients, the balance in the top left, any other screen you visit, etc. As a mental model, that seems easier track than having individual components have their own setting.

Originally posted by @GBKS in https://github.com/bitcoin-core/gui-qml/issues/450#issuecomment-2948807685