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/lib/saleor/payments/cod.ts

@@ -0,0 +1,149 @@
+/**
+ * Cash on Delivery (COD) payment logic
+ * Handles creation of COD transactions in Saleor
+ */
+
+import type { Money, TransactionInput } from '@/lib/saleor/payments/types';
+import { generateCODReference } from '@/lib/config/paymentMethods';
+
+import { gql } from "@apollo/client";
+
+/**
+ * GraphQL mutation to create a transaction on an order
+ */
+export const CREATE_TRANSACTION_MUTATION = gql`
+  mutation TransactionCreate($id: ID!, $transaction: TransactionCreateInput!) {
+    transactionCreate(id: $id, transaction: $transaction) {
+      transaction {
+        id
+        name
+        pspReference
+        status
+        availableActions
+        amountAuthorized {
+          amount
+          currency
+        }
+        amountCharged {
+          amount
+          currency
+        }
+      }
+      errors {
+        field
+        message
+        code
+      }
+    }
+  }
+`;
+
+/**
+ * Create a Cash on Delivery transaction configuration
+ * @param orderNumber - The order number for reference
+ * @param amount - The order total amount
+ * @returns TransactionInput for Saleor
+ */
+export function createCODTransactionInput(
+  orderNumber: string,
+  amount: Money
+): TransactionInput {
+  return {
+    name: 'Cash on Delivery',
+    pspReference: generateCODReference(orderNumber),
+    availableActions: ['CHARGE'],
+    amountAuthorized: {
+      amount: 0,
+      currency: amount.currency,
+    },
+    amountCharged: {
+      amount: 0,
+      currency: amount.currency,
+    },
+    externalUrl: null,
+  };
+}
+
+/**
+ * Create COD transaction on an order
+ * This should be called after checkoutComplete creates the order
+ * 
+ * @param orderId - Saleor order ID
+ * @param orderNumber - Human-readable order number
+ * @param amount - Order total amount
+ * @returns Promise with transaction result
+ */
+export async function createCODTransaction(
+  orderId: string,
+  orderNumber: string,
+  amount: Money
+): Promise<{ success: boolean; transaction?: unknown; errors?: unknown[] }> {
+  try {
+    // Note: This function should be called from a Server Component or API route
+    // as it requires making a GraphQL mutation with authentication
+    
+    const transactionInput = createCODTransactionInput(orderNumber, amount);
+    
+    // The actual GraphQL call will be made in the checkout page
+    // This function just prepares the input
+    return {
+      success: true,
+      transaction: {
+        orderId,
+        ...transactionInput,
+      },
+    };
+  } catch (error) {
+    console.error('Error creating COD transaction:', error);
+    return {
+      success: false,
+      errors: [{ message: 'Failed to create COD transaction' }],
+    };
+  }
+}
+
+/**
+ * Check if an order has a COD transaction
+ * @param order - Order object from Saleor
+ * @returns boolean
+ */
+export function hasCODTransaction(order: { transactions?: Array<{ name?: string }> }): boolean {
+  if (!order.transactions || order.transactions.length === 0) {
+    return false;
+  }
+  
+  return order.transactions.some(
+    (t) => t.name === 'Cash on Delivery'
+  );
+}
+
+/**
+ * Get COD transaction from order
+ * @param order - Order object from Saleor
+ * @returns COD transaction or undefined
+ */
+export function getCODTransaction(order: { transactions?: Array<{ name?: string }> }) {
+  if (!order.transactions) return undefined;
+  
+  return order.transactions.find(
+    (t) => t.name === 'Cash on Delivery'
+  );
+}
+
+/**
+ * Format COD status for display
+ * @param transactionStatus - Transaction status from Saleor
+ * @returns Human-readable status
+ */
+export function formatCODStatus(transactionStatus: string): string {
+  switch (transactionStatus) {
+    case 'NOT_CHARGED':
+      return 'Pending Collection';
+    case 'CHARGED':
+      return 'Paid';
+    case 'CANCELLED':
+      return 'Cancelled';
+    default:
+      return transactionStatus;
+  }
+}