Costs already resolved `default_currency || trip.currency || 'EUR'`, but the
setting could never actually be empty: the store seeded 'USD', so a user who had
never touched it silently had every trip converted into dollars, and the picker
offered no way to unset it.
Seed it empty and lead the picker with a "Trip currency" option, so an unset
preference means "show each trip in its own currency" instead of forcing them all
through one code. An explicit empty value persists and beats the admin-set
instance default — it is a deliberate choice, not an absence.
This also brings the public share's fallback to life: the share payload has
resolved sharer's currency -> trip currency since #1361, but the trip-currency
branch was unreachable while every owner had a currency forced on them.
Plugins are handed `formats.currency` as a concrete code, so PluginFrame now
resolves the same chain rather than passing an empty string through the bridge.
jubnl