feat(pos): add write-off functionality for showcase kits

Add support for writing off showcase kits by creating a write-off document with components, converting reservations, and updating statuses.

- Add `write_off_from_showcase` static method to ShowcaseManager
- Add API endpoint `/pos/api/product-kits/<int:kit_id>/write-off/`
- Add write-off button to POS terminal UI
- Implement confirmation dialog with detailed information
- Add redirect to write-off document detail page after success

The write-off process includes:
1. Creating a write-off document in draft state
2. Converting existing reservations to write-off document items
3. Marking the showcase item as dismantled
4. Setting the product kit status to discontinued (if not already)

Breaking Changes: No

myproject/pos/urls.py

@@ -33,6 +33,8 @@ urlpatterns = [
     path('api/product-kits/<int:kit_id>/update/', views.update_product_kit, name='update-product-kit'),
     # Разобрать витринный комплект (освободить резервы, установить статус discontinued) [POST]
     path('api/product-kits/<int:kit_id>/disassemble/', views.disassemble_product_kit, name='disassemble-product-kit'),
+    # Списать витринный комплект (создать документ списания с компонентами) [POST]
+    path('api/product-kits/<int:kit_id>/write-off/', views.write_off_showcase_kit, name='write-off-showcase-kit'),
     # Создать временный комплект и зарезервировать на витрину [POST]
     path('api/create-temp-kit/', views.create_temp_kit_to_showcase, name='create-temp-kit-api'),
     # Создать заказ и провести оплату в POS [POST]