Add a global option to choose the default currency/exchange rate direction

This is a continuation of

https://budgetflow.featurebase.app/p/transfers-cross-currency-are-still-to-improve

Thanks, the implementation works perfectly!

One more idea around it is to add a global setting to choose, which currency/exchange rate direction to show by default here:

and here:

For example, I would choose to show “From” as incoming currency by default (considering USD→UAH exchange, this would be USD instead of currently chosen UAH), “To” as account’s currency (“UAH” in this case"), and the Exchange Rate would be 49,7512 instead of 0,0201.

Like here, just by default:

But the app has already become so much better in 3.1.0, this is not so important :)