Currency Swapper

[cjeria]

Inputting an exact fiat value with ETH is difficult. Let's add a swapper to the amount field in the send screen.

[alextsg]

@cjeria Now that we have the currency toggle in the Settings page, do we still need this?

[cjeria]

Let's keep it and use it to measure how people calculate amounts they want to send. This could help us understand how people think (do they think in Fiat, or do they think in ETH?)

The default here could be set to whatever the user set the main currency in global settings, and they can freely toggle the currency here without it affecting the main currency that they set in global settings.

[whymarrh]

Fixed with #6091