feat(promo-code): complete promo code feature implementation

- Add promo_code_enabled field to SystemSettings and PublicSettings DTOs
- Add promo code validation in registration flow
- Add admin settings UI for promo code configuration
- Add i18n translations for promo code feature

frontend/src/i18n/locales/en.ts

@@ -2726,7 +2726,9 @@ export default {
         enableRegistration: 'Enable Registration',
         enableRegistrationHint: 'Allow new users to register',
         emailVerification: 'Email Verification',
-        emailVerificationHint: 'Require email verification for new registrations'
+        emailVerificationHint: 'Require email verification for new registrations',
+        promoCode: 'Promo Code',
+        promoCodeHint: 'Allow users to use promo codes during registration'
       },
       turnstile: {
         title: 'Cloudflare Turnstile',