implement empty-container return service: add return quantity handling, update related DTOs, services, and UI components

packages/types/src/freight/contracts.ts

@@ -733,6 +733,11 @@ export interface CreateBookingContainerLineDto {
   quantity: number;
   hazardousQuantity?: number;
   reeferQuantity?: number;
+  /**
+   * How many units ship with empty-container return (≤ quantity). Only allowed
+   * when the contract was created WITH_RETURN (container freight only).
+   */
+  returnQuantity?: number;
   units: CreateContainerUnitDto[];
 }