docs: update Letzshop order import documentation

Add documentation for:
- Item-level confirmation workflow (per Letzshop API requirements)
- Terminology update (rejected -> declined)
- Historical import of declined orders

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/implementation/letzshop-order-import-improvements.md

@@ -439,6 +439,34 @@ From the Letzshop merchant interface:
 - **Findings**: Bug is on Letzshop's side, no client-side workaround possible
 - **Recommendation**: Store tracking info locally after setting via API
 
+### Item-Level Confirmation ✅
+- **Issue**: Orders were being confirmed/declined at order level, but Letzshop requires item-level actions
+- **Letzshop Model**:
+  - Each `inventoryUnit` must be confirmed/declined individually via `confirmInventoryUnits` mutation
+  - `isAvailable: true` = confirmed, `isAvailable: false` = declined
+  - Inventory unit states: `unconfirmed` → `confirmed_available` / `confirmed_unavailable` / `returned`
+  - Shipment states derived from items: `unconfirmed` / `confirmed` / `declined`
+  - Partial confirmation allowed (some items confirmed, some declined)
+- **Fix**:
+  1. Order detail modal now shows each item with product details (name, EAN, SKU, MPN, price)
+  2. Per-item confirm/decline buttons for pending items
+  3. Admin API endpoints for single-item and bulk operations:
+     - `POST /vendors/{id}/orders/{id}/items/{id}/confirm`
+     - `POST /vendors/{id}/orders/{id}/items/{id}/decline`
+  4. Order status automatically updates based on item states:
+     - All items declined → order status = "declined"
+     - Any item confirmed → order status = "confirmed"
+
+### Terminology Update ✅
+- Changed "Rejected" to "Declined" throughout UI to match Letzshop terminology
+- Internal `sync_status` value remains "rejected" for backwards compatibility
+- Filter dropdown, status badges, and action buttons now use "Declined"
+- Added "Declined" stats card to orders dashboard
+
+### Historical Import: Declined Orders ✅
+- Historical import now fetches both `confirmed` AND `declined` shipments
+- Combined stats shown in import result
+
 ---
 
 ## Next Steps (TODO)