fix: use lowercase 'cod' for translation keys to match method ID

- Change translation keys from COD to cod in all locale files
- Fixes MISSING_MESSAGE error in PaymentMethodCard
- Aligns translation keys with payment method IDs

src/i18n/messages/en.json

@@ -439,7 +439,7 @@
     "securePayment": "Secure payment processing",
     "noMethodsAvailable": "No payment methods available",
     "singleMethodNotice": "Cash on Delivery is the only available payment method for your location",
-    "COD": {
+    "cod": {
       "name": "Cash on Delivery",
       "description": "Pay when you receive your order",
       "instructions": {