The trip currency is the base every expense and settle-up is netted against, but
the only picker for it lived in the legacy Budget addon panel — so on the Costs
panel a trip was stuck with whatever it was created as. Add the field to the trip
form, on create and on edit, gated on trip_edit. The REST and MCP write paths
already accepted `currency`; only the form was missing.
Changing it is not a rename, though: an expense's frozen `exchange_rate` is
"units of its currency per 1 trip currency", and `currency = NULL` means "the
trip's own", so both are relative to the outgoing base. Swapping it out from
under them redenominates the implicit rows (9 000 RUB becoming 9 000 EUR) and
leaves the frozen rates pointing at a currency the trip no longer uses — the same
mismatch that inflated #1543.
So rebaseTripCurrency() runs first, while the old currency is still on the row:
it pins the implicit rows to the outgoing currency and re-freezes every rate
against the incoming one, for expenses and settle-up transfers alike. No stored
amount is rewritten — each keeps the figure the user typed, in the currency they
typed it in, and its real-world value survives the switch.
Also covers the #1543 data as a settlement regression test.
jubnl
900658de0a