feat(warehouses): backoffice UI for container stack positions

Surfaces the physical stack/slot model in the staff app.

- ZoneLayoutModal: stacks drawn level by level with occupancy colours,
  configured-vs-built-vs-occupied counts, and stack create/delete plus
  block/reserve/free on empty levels
- SlotPicker in the store and move modals, offering only the next
  fillable level of each stack so the form cannot suggest a position
  the API will refuse
- move modal warns when a container is buried, lists the blockers, and
  disables the action instead of firing a 409
- fix: move() now asserts accessibility server-side, matching release —
  both are exits from a stack

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

@@ -7,6 +7,7 @@ import {
   IsEmail,
   IsIn,
   IsInt,
+  IsNotEmpty,
   IsNumber,
   IsOptional,
   IsString,
@@ -32,10 +33,12 @@ export class CreateContainerUnitDto {
   })
   containerNumber!: string;
 
-  @ApiPropertyOptional()
-  @IsOptional()
+  @ApiProperty({ description: 'Seal number — required on every container, import and export alike.' })
   @IsString()
-  sealNumber?: string;
+  @Transform(({ value }) => (typeof value === 'string' ? value.trim() : value))
+  @IsNotEmpty({ message: 'sealNumber is required' })
+  @MaxLength(64)
+  sealNumber!: string;
 
   @ApiProperty({ description: 'VGM in tons', minimum: 0 })
   @IsNumber()