fix: add required email and country fields to checkout

- Add email field (required) for order confirmation
- Add phone field in contact info section
- Add country dropdown with regional options
- Add validation for email format and required fields
- Add checkoutEmailUpdate mutation call before completing
- Use selected country instead of hardcoded RS
- Add translations for new fields (EN, SR, DE, FR)

src/i18n/messages/en.json

@@ -386,7 +386,12 @@
   },
   "Checkout": {
     "checkout": "Checkout",
+    "contactInfo": "Contact Information",
+    "email": "Email",
+    "emailRequired": "Required for order confirmation",
+    "phoneRequired": "Required for delivery coordination",
     "shippingAddress": "Shipping Address",
+    "country": "Country",
     "firstName": "First Name",
     "lastName": "Last Name",
     "streetAddress": "Street Address",
@@ -410,6 +415,8 @@
     "yourCartEmpty": "Your cart is empty",
     "continueShopping": "Continue Shopping",
     "errorNoCheckout": "No active checkout. Please try again.",
+    "errorEmailRequired": "Please enter a valid email address.",
+    "errorFieldsRequired": "Please fill in all required fields.",
     "errorOccurred": "An error occurred during checkout.",
     "errorCreatingOrder": "Failed to create order.",
     "orderConfirmed": "Order Confirmed!",