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/contract-booking.service.ts

@@ -2250,7 +2250,9 @@ export class ContractBookingService {
           unitRepo.create({
             bookingContainerId: containerRow.id,
             containerNumber: unit.containerNumber,
-            sealNumber: unit.sealNumber ?? null,
+            // Legacy units recovered by the remainder placement can still
+            // arrive sealless — keep those null rather than empty-string.
+            sealNumber: unit.sealNumber?.trim() || null,
             vgmTons: unit.vgmTons,
             isHazardous: unit.isHazardous ?? false,
             isReefer: unit.isReefer ?? false,