Feature request: Support different currencies for balances

[vbrandl]

What's the feature 🧐

This might be a bigger request than my last two, but it would be nice to be able to add balances in different currencies. From what I've seen, the current implementation assumes everything is in the same (undefined) currency and the actual currency symbol is derived from the language preferences (which is kinda confusing when using KitchenOwl on multiple devices with different language preferences).

This feature would be even nicer, if it included a way to convert between currencies using the exchange rate of the day the balance was added.

Extra information and references

• The ECB seems to publish exchange rates including historical data: https://www.ecb.europa.eu/stats/policy_and_exchange_rates/euro_reference_exchange_rates/html/index.en.html