fix: Add SALEOR_API_TOKEN auth for COD transaction creation

- Add SALEOR_API_TOKEN environment variable support
- Update Apollo client to include auth header
- Enable COD transaction creation after checkout

src/lib/saleor/mutations/Checkout.ts

@@ -210,3 +210,17 @@ export const CHECKOUT_LANGUAGE_CODE_UPDATE = gql`
     }
   }
 `;
+
+export const TRANSACTION_CREATE = gql`
+  mutation CreateTransaction($orderId: ID!, $transaction: TransactionCreateInput!) {
+    transactionCreate(id: $orderId, transaction: $transaction) {
+      transaction {
+        id
+      }
+      errors {
+        field
+        message
+      }
+    }
+  }
+`;