Fix PayPal MULTI_CURRENCY_ORDER error by using consistent GBP

All store items are priced in GBP (filtered by price_gbp > 0).
Hardcode GBP currency throughout the checkout and order creation
flow to prevent currency mismatches between the PayPal SDK, order
items, and purchase unit amounts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/app/api/orders/create/route.ts

@@ -59,8 +59,9 @@ export async function POST(request: NextRequest) {
     }
 
     // Calculate total with shipping
+    // All store items are priced in GBP (store filters by price_gbp > 0)
     const subtotal = items.reduce((sum, item) => sum + item.price * item.quantity, 0);
-    const currency = items[0].currency;
+    const currency = 'GBP';
     const shippingCost = getShippingCost(shipping.country, currency);
     const total = subtotal + shippingCost;
 
@@ -94,11 +95,11 @@ export async function POST(request: NextRequest) {
       })),
     );
 
-    // Create PayPal order
+    // Create PayPal order — all items use the same currency
     const paypalItems: PayPalItem[] = items.map(item => ({
       name: item.artworkName,
       unit_amount: {
-        currency_code: item.currency,
+        currency_code: currency,
         value: item.price.toFixed(2),
       },
       quantity: String(item.quantity),

frontend/src/app/checkout/page.tsx

@@ -45,8 +45,9 @@ export default function CheckoutPage() {
     setFormValid(required.every(f => updated[f].trim() !== ''));
   };
 
-  const currency = items[0]?.artwork?.currency || 'GBP';
-  const currencySymbol = currency === 'GBP' ? '\u00a3' : '$';
+  // All store items are priced in GBP
+  const currency = 'GBP';
+  const currencySymbol = '\u00a3';
   const shippingCost = getShippingCost(formData.country, currency);
   const total = subtotal + shippingCost;
 
@@ -54,7 +55,7 @@ export default function CheckoutPage() {
     artworkId: parseInt(item.id),
     artworkName: item.title,
     price: item.price,
-    currency: item.artwork.currency || 'GBP',
+    currency: 'GBP',
     quantity: item.quantity,
   }));