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

apps/edr-freight-api/src/modules/contracts/dto/create-booking-under-contract.dto.ts

@@ -77,6 +77,18 @@ export class CreateBookingContainerLineDto {
   @Transform(({ value }) => Number(value))
   reeferQuantity?: number;
 
+  @ApiPropertyOptional({
+    minimum: 0,
+    description:
+      'How many units of this line ship with empty-container return (≤ quantity). ' +
+      'Only allowed when the contract was created WITH_RETURN (container freight).',
+  })
+  @IsOptional()
+  @IsInt()
+  @Min(0)
+  @Transform(({ value }) => Number(value))
+  returnQuantity?: number;
+
   @ApiProperty({ type: [CreateContainerUnitDto] })
   @IsArray()
   @ValidateNested({ each: true })