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/views/admin/SettingsView.vue

@@ -323,6 +323,21 @@
               </div>
               <Toggle v-model="form.email_verify_enabled" />
             </div>
+
+            <!-- Promo Code -->
+            <div
+              class="flex items-center justify-between border-t border-gray-100 pt-4 dark:border-dark-700"
+            >
+              <div>
+                <label class="font-medium text-gray-900 dark:text-white">{{
+                  t('admin.settings.registration.promoCode')
+                }}</label>
+                <p class="text-sm text-gray-500 dark:text-gray-400">
+                  {{ t('admin.settings.registration.promoCodeHint') }}
+                </p>
+              </div>
+              <Toggle v-model="form.promo_code_enabled" />
+            </div>
           </div>
         </div>
 
@@ -1013,6 +1028,7 @@ type SettingsForm = SystemSettings & {
 const form = reactive<SettingsForm>({
   registration_enabled: true,
   email_verify_enabled: false,
+  promo_code_enabled: true,
   default_balance: 0,
   default_concurrency: 1,
   site_name: 'Sub2API',
@@ -1135,6 +1151,7 @@ async function saveSettings() {
     const payload: UpdateSettingsRequest = {
       registration_enabled: form.registration_enabled,
       email_verify_enabled: form.email_verify_enabled,
+      promo_code_enabled: form.promo_code_enabled,
       default_balance: form.default_balance,
       default_concurrency: form.default_concurrency,
       site_name: form.site_name,