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/bookings/booking-wagon-cancellation.service.ts

@@ -1875,9 +1875,12 @@ export class BookingWagonCancellationService {
           // the same cargo); number/seal/VGM come from the override when given.
           units: sized.map((u, i) => ({
             containerNumber: replacement?.[i]?.containerNumber ?? u.containerNumber,
+            // A credit snapshot taken before seals were mandatory can carry
+            // none; the booking service normalizes the blank back to null
+            // rather than blocking the rebook of already-paid cargo.
             sealNumber: replacement
-              ? (replacement[i]?.sealNumber ?? undefined)
-              : (u.sealNumber ?? undefined),
+              ? (replacement[i]?.sealNumber ?? '')
+              : (u.sealNumber ?? ''),
             vgmTons: replacement?.[i]?.vgmTons ?? u.vgmTons,
             isHazardous: u.isHazardous,
             isReefer: u.isReefer,