The default currency is what a bare amount means — 25 groceries cash
records 25 of it. It is also what lands in the expense category when a foreign card pays a local bill.
Listing currencies turns on enforcement: anything outside the list is refused rather than
splitting a category across two currencies that no report will combine without a rate. Leave it empty to accept any.
The timezone decides what today is. An evening entry recorded in the wrong zone
lands on the wrong day, and nothing later can tell.