fix(rates): render stored rate currency, default last mile to birr

The rate matrix currency cell hardcoded USD, so a last-mile rate
priced in ETB still displayed as dollars. formatCell now takes the
row and reads its currency code, falling back to USD.

Last-mile currency select gets defaultValue ETB (new generic
FormFieldDef.defaultValue for create-time pre-selection) and lists
ETB (Birr) first; USD stays selectable. Backend already persisted
and validated the chosen currency.

apps/edr-freight-web/backoffice/src/components/ruleEngine/RuleEngineFormDialog.tsx

@@ -127,6 +127,8 @@ const buildInitialValues = (
       } else {
         values[field.name] = raw;
       }
+    } else if (field.defaultValue !== undefined) {
+      values[field.name] = field.defaultValue;
     } else if (field.type === "boolean") {
       values[field.name] = false;
     } else if (field.type === "number") {