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/templates/pos/terminal.html

@@ -330,6 +330,11 @@
           <i class="bi bi-scissors"></i> Разобрать букет
         </button>
 
+        <!-- Кнопка "Списать" (отображается только в режиме редактирования) -->
+        <button type="button" class="btn btn-warning me-auto" id="writeOffKitBtn" style="display: none;">
+          <i class="bi bi-file-earmark-x"></i> Списать букет
+        </button>
+
         <!-- Правая группа кнопок -->
         <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Отмена</button>
         <button type="button" class="btn btn-primary" id="confirmCreateTempKit">