Amount does not update when exchange rate is edited

[IzzDarki]

1. Create an expense in a currency other than the main currency of the group with a custom exchange rate
2. Save
3. Open it again and edit the exchange rate
4. Save

Now the expense is saved with the new exchange rate, but the amount in the main currency did not change, i.e., the updated exchange rate is not applied, even though it looks like that for the user.

Current workaround: Re-enter the amount in the foreign currency, and the amount in the main currency updates instantly.

(tested on current version of https://spliit.app)