feat: add 30-day money back guarantee trust badge above complete order button

- Add green trust badge with checkmark icon above 'Complete Order' button
- Add translations for all locales (EN, SR, DE, FR)
- Badge includes: '30-Day Money-Back Guarantee' text
- Styled with green background and border to match trust/conversion theme

src/i18n/messages/sr.json

@@ -407,6 +407,7 @@
     "cashOnDeliveryDesc": "Platite kada vam narudžbina bude isporučena na vrata.",
     "processing": "Obrađivanje...",
     "completeOrder": "Završi narudžbinu - {total}",
+    "moneyBackGuarantee": "30-dnevna garancija povrata novca",
     "orderSummary": "Pregled narudžbine",
     "qty": "Kol",
     "subtotal": "Ukupno",