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
ff481f18c3
