[Suggestion] Show foreign currency when creating expense transaction

[etnperlong]

Hi,

Thanks for your work fixing issue #95. This fix has made it much easier for me to record cross-border money transfers when paying off credit cards.

As I continue to use Pico to keep track of my foreign currency card (in HKD) purchases locally (by GBP), I found that there is no option for me to record the price in foreign currency.

I'd like to describe my usage scenario because in Europe/UK, the rewards for using a credit card are relatively low (usually only around 0.75% ~1.5%), but if I use a Hong Kong/US credit card, I can get around 4 ~ 5% cashback. So I tend to use Hong Kong/US credit cards in the UK to get higher rewards.

However, there are some FTF (Foreign Transaction Fee) charges, which are around 1.95% when I use these cards. This purchase (including this charge) will be recorded in the currency (HKD/USD) of the credit card. Therefore, I would like to record both the local currency (GBP) and the foreign currency amount (HKD) when I record these purchases, and use it to calculate the actual rewards rate I get when I pay my bill every month.

Pico currently lacks such a feature, I can only record the foreign currency (HKD) at the time of purchase and use the original Firefly III's UI to complete the information.

Since I don't know how to develop with Vue (Actually, I only know React 😂), I hope you will consider my needs and add such a feature, I will be eternally grateful! Thanks!