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

backend/internal/service/settings_view.go

@@ -3,6 +3,7 @@ package service
 type SystemSettings struct {
 	RegistrationEnabled bool
 	EmailVerifyEnabled  bool
+	PromoCodeEnabled    bool
 
 	SMTPHost               string
 	SMTPPort               int
@@ -58,6 +59,7 @@ type SystemSettings struct {
 type PublicSettings struct {
 	RegistrationEnabled bool
 	EmailVerifyEnabled  bool
+	PromoCodeEnabled    bool
 	TurnstileEnabled    bool
 	TurnstileSiteKey    string
 	SiteName            string