fix(po): store the price from the form so POs stop printing £0.00

Every purchase order created from a stock item printed a £0.00 unit
price, even though a cost price was on the record and the form showed
the right number.

Cause: the PO snapshot took costPriceGBP from the stock item's stored
cost_price_gbp instead of from the form. The price the user sees (and
types) into the "Unit Price (£ GBP)" field landed in snapshot.costPrice,
which nothing renders, while the PO PDF renders snapshot.costPriceGBP
(pdf.ts: `£${item.costPriceGBP}`). So a price typed into the form was
discarded and the stock item's cached value was printed instead.

Because the stock item's cost_price_gbp was only written back to the
database AFTER the PO was created, the first PO of an item always showed
£0.00 and only the next one showed the price. Verified in production:
PO-20261023-002 stored costPrice 333.78 / costPriceGBP 0 and printed
£0.00, while the linked stock item's cost_price_gbp was 0 at the time
and was updated to 333.78 by the sync step three minutes later.

Fixes:
- PurchaseOrderForm: snapshot.costPriceGBP is now the price from the
  form; snapshot.costPrice is the stock item's ZAR landed cost.
- PurchaseOrderForm: selecting a stock item with no GBP cost price
  leaves the price field blank instead of showing £0.00, with a hint
  that the supplier's GBP price still has to be entered. Most stock
  items only carry a ZAR cost price, so £0.00 looked like a real price.
- EditPurchaseOrderDialog: preserve the ZAR cost price per row instead
  of zeroing it when a PO is edited.

src/components/purchase-orders/EditPurchaseOrderDialog.tsx

@@ -134,13 +134,18 @@ export function EditPurchaseOrderDialog({ open, onOpenChange, order, refreshData
       }))
 
       // Build updated snapshot — keep stable ids + entry mode so the audit
-      // trail remains meaningful.
+      // trail remains meaningful. costPrice is the ZAR landed cost, so carry
+      // the existing value over per row instead of zeroing it; costPriceGBP is
+      // the price the user sees in the form and is what the PO PDF renders.
+      const existingSnapshot = order.snapshotItems || []
       const updatedSnapshot = normalizedRows.map((r) => ({
         stockItemId: r.stockItemId,
         itemName: r.itemName,
         sku: r.sku,
         quantity: r.quantity,
-        costPrice: 0,
+        costPrice: Number(
+          existingSnapshot.find((s) => s.stockItemId === r.stockItemId)?.costPrice || 0,
+        ),
         costPriceGBP: r.unitPriceGBP,
         entryMode: r.entryMode,
       }))

src/components/purchase-orders/PurchaseOrderForm.tsx

@@ -11,6 +11,7 @@ import { ToggleGroup, ToggleGroupItem } from "@/components/ui/toggle-group"
 import {
   Form,
   FormControl,
+  FormDescription,
   FormField,
   FormItem,
   FormLabel,
@@ -260,8 +261,12 @@ export function PurchaseOrderForm({ setOpen, refreshData }: PurchaseOrderFormPro
       form.setValue(`items.${index}.stockItemId`, selectedStockItem.id, { shouldDirty: true });
       form.setValue(`items.${index}.itemName`, selectedStockItem.name, { shouldDirty: true });
       form.setValue(`items.${index}.sku`, selectedStockItem.sku, { shouldDirty: true });
-      // Use GBP price for purchase orders
-      form.setValue(`items.${index}.costPrice`, selectedStockItem.costPriceGBP, { shouldDirty: true, shouldValidate: true });
+      // Use GBP price for purchase orders. Most stock items only carry a ZAR
+      // cost price (cost_price), so leave the field BLANK instead of silently
+      // showing £0.00 — a blank field tells the user the supplier's GBP price
+      // still has to be entered, whereas "£0.00" looks like a real price.
+      const gbpPrice = Number(selectedStockItem.costPriceGBP || 0);
+      form.setValue(`items.${index}.costPrice`, gbpPrice > 0 ? gbpPrice : ("" as any), { shouldDirty: true, shouldValidate: true });
       // Leave quantity blank — user types the amount they want.
       form.setValue(`items.${index}.quantity`, "", { shouldDirty: true, shouldValidate: true });
     }
@@ -357,8 +362,14 @@ export function PurchaseOrderForm({ setOpen, refreshData }: PurchaseOrderFormPro
           itemName: item.itemName,
           sku: item.sku,
           quantity: item.quantity,
-          costPrice: Number(item.costPrice || 0),
-          costPriceGBP: Number(stockItem?.costPriceGBP || 0),
+          // ZAR landed cost, for reference only — the PO itself is priced in GBP.
+          costPrice: Number(stockItem?.costPrice ?? 0),
+          // The GBP unit price taken from the FORM. This is the only price
+          // field the PO PDF renders (£${item.costPriceGBP}), so it must be the
+          // value the user sees and edits. Reading it from the stock item
+          // instead silently discarded every price typed into the form and
+          // printed £0.00 on the PO.
+          costPriceGBP: Number(item.costPrice || 0),
           entryMode: 'stock',
         });
       }
@@ -755,6 +766,11 @@ export function PurchaseOrderForm({ setOpen, refreshData }: PurchaseOrderFormPro
                               onChange={(v) => field.onChange(v)}
                             />
                           </FormControl>
+                          {!(Number(field.value || 0) > 0) && (
+                            <FormDescription className="text-xs text-amber-600 dark:text-amber-400">
+                              No GBP cost price on this stock item — enter the supplier&apos;s GBP price.
+                            </FormDescription>
+                          )}
                           <FormMessage />
                         </FormItem>
                       )}