fix(pos): restrict quantity editing for showcase kits in cart edit modal

For showcase kits (showcase_kit type), the quantity field is now disabled
in the cart item edit modal since these are pre-assembled physical items
with reservations. Price editing remains available.

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

myproject/pos/templates/pos/components/edit_cart_item_modal.html

@@ -42,6 +42,9 @@
           <label for="editModalQuantity" class="form-label fw-semibold">Количество</label>
           <input type="number" class="form-control" id="editModalQuantity"
                  min="0.001" step="0.001" value="1">
+          <div id="editModalQtyHint" class="text-muted small mt-1" style="display: none;">
+            <i class="bi bi-info-circle"></i> Количество нельзя изменить для витринного комплекта (собранный товар с резервами)
+          </div>
         </div>
 
         <!-- Итого -->