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/api/admin/settings.ts

@@ -12,6 +12,7 @@ export interface SystemSettings {
   // Registration settings
   registration_enabled: boolean
   email_verify_enabled: boolean
+  promo_code_enabled: boolean
   // Default settings
   default_balance: number
   default_concurrency: number
@@ -64,6 +65,7 @@ export interface SystemSettings {
 export interface UpdateSettingsRequest {
   registration_enabled?: boolean
   email_verify_enabled?: boolean
+  promo_code_enabled?: boolean
   default_balance?: number
   default_concurrency?: number
   site_name?: string