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/config/paymentMethods.ts

@@ -0,0 +1,106 @@
+/**
+ * Payment methods configuration
+ * Centralized configuration for all available payment methods
+ */
+
+import type { PaymentMethod, Money } from '@/lib/saleor/payments/types';
+
+/**
+ * List of all available payment methods
+ * Configure availability per channel, fees, and other settings
+ */
+export const paymentMethods: PaymentMethod[] = [
+  {
+    id: 'cod',
+    name: 'Cash on Delivery',
+    description: 'Pay when you receive your order',
+    type: 'simple',
+    fee: 0,
+    available: true,
+    availableInChannels: ['default-channel'], // Currently Serbia only
+    icon: 'Banknote',
+  },
+  {
+    id: 'card',
+    name: 'Credit Card',
+    description: 'Secure online payment',
+    type: 'app',
+    fee: 0,
+    available: false, // Coming soon
+    availableInChannels: ['default-channel'],
+    icon: 'CreditCard',
+  },
+  {
+    id: 'bank_transfer',
+    name: 'Bank Transfer',
+    description: 'Pay via bank transfer',
+    type: 'simple',
+    fee: 0,
+    available: false, // Coming later
+    availableInChannels: ['default-channel'],
+    icon: 'Building2',
+  },
+];
+
+/**
+ * Get payment methods available for a specific channel
+ */
+export function getPaymentMethodsForChannel(channel: string): PaymentMethod[] {
+  return paymentMethods.filter(
+    (method) =>
+      method.available && method.availableInChannels.includes(channel)
+  );
+}
+
+/**
+ * Get a specific payment method by ID
+ */
+export function getPaymentMethodById(id: string): PaymentMethod | undefined {
+  return paymentMethods.find((method) => method.id === id);
+}
+
+/**
+ * Check if a payment method is available for a channel
+ */
+export function isPaymentMethodAvailable(
+  methodId: string,
+  channel: string
+): boolean {
+  const method = getPaymentMethodById(methodId);
+  if (!method) return false;
+  return method.available && method.availableInChannels.includes(channel);
+}
+
+/**
+ * Default payment method ID
+ * Used when no payment method is explicitly selected
+ */
+export const DEFAULT_PAYMENT_METHOD = 'cod';
+
+/**
+ * Channel configuration
+ * Maps channels to their supported payment methods
+ */
+export const channelPaymentConfig: Record<string, string[]> = {
+  'default-channel': ['cod'], // Serbia - COD only for now
+};
+
+/**
+ * Format payment method fee for display
+ */
+export function formatPaymentFee(fee: number, currency: string): string {
+  if (fee === 0) return 'No additional fee';
+  return new Intl.NumberFormat('en-US', {
+    style: 'currency',
+    currency: currency,
+  }).format(fee);
+}
+
+/**
+ * Generate PSP reference for COD transactions
+ * Format: COD-{orderNumber}-{timestamp}
+ */
+export function generateCODReference(orderNumber: string): string {
+  const timestamp = Date.now();
+  return `COD-${orderNumber}-${timestamp}`;
+}