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/warehouses/warehouse-inventory.service.ts

@@ -3140,6 +3140,13 @@ export class WarehouseInventoryService {
 
       const { warehouse, yard, zone } = await this.validateLocation(manager, dto);
 
+      // Taking a box out of a stack is physically impossible while others stand
+      // on top of it — the same rule the release path enforces. Moving is one of
+      // those exits, so it is checked here rather than only at release.
+      if (item.slotId) {
+        await this.placement.assertAccessible(item.id, manager);
+      }
+
       // A move that names a slot is validated against the hierarchy it claims;
       // one that does not clears the old slot, because the box has left it.
       if (dto.slotId) {