feat(ui-common): shared currency formatting + DJF in CurrencySelector

New lib/currency.ts is the single source of truth for currency
display across both freight web apps: symbol, name and decimals per
code (DJF is zero-decimal by convention, unlike ETB/USD). Portal's own
currency lib and backoffice's ad-hoc formatters are rewired onto this
in the next two commits instead of each guessing decimals.

CurrencySelector gets an allowDjf prop mirroring the existing allowUsd
gate, so import-shipment currency pickers can offer DJF the same way
they offer USD.

Claude-Session: https://claude.ai/code/session_01CZy77vCWhka3pnmVF9NDkL

packages/ui-common/src/index.ts

@@ -80,3 +80,12 @@ export type {
   BookingWindowStateInput,
   BookingWindowUiState,
 } from "./lib/booking-window-display";
+
+export {
+  CURRENCY_META,
+  CURRENCY_CODES,
+  currencyDecimals,
+  currencySymbol,
+  formatCurrency,
+} from "./lib/currency";
+export type { SupportedCurrency } from "./lib/currency";