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
Andrey Smakotin
38fbf36731