feat: implement Cash on Delivery (COD) payment method

This commit adds comprehensive COD support using Saleor's native Transaction system:

**Architecture:**
- Uses Saleor's native Transaction objects (not metadata)
- Modular payment method configuration
- Extensible design for future payment types (cards, bank transfer, etc.)

**New Components:**
- PaymentMethodSelector: Reusable payment method selection UI
- PaymentMethodCard: Individual payment method card
- CODInstructions: COD-specific instructions and guidance
- PaymentSection: Checkout integration wrapper

**Core Features:**
- COD selected by default for Serbia (default-channel)
- Transaction created automatically on order completion
- Transaction visible in Saleor Dashboard
- Multi-language support (EN, SR, DE, FR)
- No additional fees
- Instructions shown to customer (prepare cash, inspect order, no fee)

**Files Added:**
- docs/COD-IMPLEMENTATION-PLAN.md
- src/lib/config/paymentMethods.ts
- src/lib/saleor/payments/types.ts
- src/lib/saleor/payments/cod.ts
- src/components/payment/PaymentMethodSelector.tsx
- src/components/payment/PaymentMethodCard.tsx
- src/components/payment/CODInstructions.tsx
- src/components/payment/index.ts
- src/app/[locale]/checkout/components/PaymentSection.tsx

**Files Modified:**
- src/app/[locale]/checkout/page.tsx (added payment section, transaction creation)
- src/i18n/messages/{en,sr,de,fr}.json (payment translations)

**Technical Details:**
- Transaction status: NOT_CHARGED
- Available actions: [CHARGE]
- PSP Reference format: COD-{orderNumber}-{timestamp}
- Staff collects cash and fulfills order via Dashboard

Closes: Cash on Delivery payment implementation

src/app/[locale]/checkout/page.tsx

@@ -19,6 +19,9 @@ import {
   CHECKOUT_METADATA_UPDATE,
   CHECKOUT_SHIPPING_METHOD_UPDATE,
 } from "@/lib/saleor/mutations/Checkout";
+import { CREATE_TRANSACTION_MUTATION } from "@/lib/saleor/payments/cod";
+import { PaymentSection } from "./components/PaymentSection";
+import { DEFAULT_PAYMENT_METHOD } from "@/lib/config/paymentMethods";
 import { GET_CHECKOUT_BY_ID } from "@/lib/saleor/queries/Checkout";
 import type { Checkout } from "@/types/saleor";
 
@@ -38,7 +41,7 @@ interface BillingAddressUpdateResponse {
 
 interface CheckoutCompleteResponse {
   checkoutComplete?: {
-    order?: { number: string };
+    order?: { id: string; number: string };
     errors?: Array<{ message: string }>;
   };
 }
@@ -104,6 +107,7 @@ export default function CheckoutPage() {
   const [orderNumber, setOrderNumber] = useState<string | null>(null);
 
   const [sameAsShipping, setSameAsShipping] = useState(true);
+  const [selectedPaymentMethod, setSelectedPaymentMethod] = useState<string>(DEFAULT_PAYMENT_METHOD);
   const [shippingAddress, setShippingAddress] = useState<AddressForm>({
     firstName: "",
     lastName: "",
@@ -277,6 +281,11 @@ export default function CheckoutPage() {
       return;
     }
 
+    if (!selectedPaymentMethod) {
+      setError(t("errorSelectPayment"));
+      return;
+    }
+
     setIsLoading(true);
     setError(null);
 
@@ -377,6 +386,37 @@ export default function CheckoutPage() {
         const order = completeResult.data?.checkoutComplete?.order;
         if (order) {
           setOrderNumber(order.number);
+          
+          // Create COD transaction for the order
+          if (selectedPaymentMethod === 'cod') {
+            console.log("Step 6: Creating COD transaction...");
+            try {
+              await saleorClient.mutate({
+                mutation: CREATE_TRANSACTION_MUTATION,
+                variables: {
+                  id: order.id,
+                  transaction: {
+                    name: "Cash on Delivery",
+                    pspReference: `COD-${order.number}-${Date.now()}`,
+                    availableActions: ["CHARGE"],
+                    amountAuthorized: {
+                      amount: 0,
+                      currency: "RSD"
+                    },
+                    amountCharged: {
+                      amount: 0,
+                      currency: "RSD"
+                    }
+                  }
+                }
+              });
+              console.log("Step 6: COD transaction created successfully");
+            } catch (txError) {
+              console.error("Failed to create COD transaction:", txError);
+              // Don't fail the order if transaction creation fails
+            }
+          }
+          
           setOrderComplete(true);
           
           // Track order completion
@@ -660,6 +700,15 @@ export default function CheckoutPage() {
                     )}
                   </div>
 
+                  {/* Payment Method Section */}
+                  <PaymentSection
+                    selectedMethodId={selectedPaymentMethod}
+                    onSelectMethod={setSelectedPaymentMethod}
+                    locale={locale}
+                    channel="default-channel"
+                    disabled={isLoading}
+                  />
+
                   <button
                     type="submit"
                     disabled={isLoading || lines.length === 0 || !selectedShippingMethod}