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/types/index.ts

@@ -70,6 +70,7 @@ export interface SendVerifyCodeResponse {
 export interface PublicSettings {
   registration_enabled: boolean
   email_verify_enabled: boolean
+  promo_code_enabled: boolean
   turnstile_enabled: boolean
   turnstile_site_key: string
   site_name: string