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

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',

frontend/src/i18n/locales/zh.ts

@@ -2879,7 +2879,9 @@ export default {
         enableRegistration: '开放注册',
         enableRegistrationHint: '允许新用户注册',
         emailVerification: '邮箱验证',
-        emailVerificationHint: '新用户注册时需要验证邮箱'
+        emailVerificationHint: '新用户注册时需要验证邮箱',
+        promoCode: '优惠码',
+        promoCodeHint: '允许用户在注册时使用优惠码'
       },
       turnstile: {
         title: 'Cloudflare Turnstile',

frontend/src/stores/app.ts

@@ -312,6 +312,7 @@ export const useAppStore = defineStore('app', () => {
       return {
         registration_enabled: false,
         email_verify_enabled: false,
+        promo_code_enabled: true,
         turnstile_enabled: false,
         turnstile_site_key: '',
         site_name: siteName.value,

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

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,

frontend/src/views/auth/RegisterView.vue

@@ -96,7 +96,7 @@
         </div>
 
         <!-- Promo Code Input (Optional) -->
-        <div>
+        <div v-if="promoCodeEnabled">
           <label for="promo_code" class="input-label">
             {{ t('auth.promoCodeLabel') }}
             <span class="ml-1 text-xs font-normal text-gray-400 dark:text-dark-500">({{ t('common.optional') }})</span>
@@ -260,6 +260,7 @@ const showPassword = ref<boolean>(false)
 // Public settings
 const registrationEnabled = ref<boolean>(true)
 const emailVerifyEnabled = ref<boolean>(false)
+const promoCodeEnabled = ref<boolean>(true)
 const turnstileEnabled = ref<boolean>(false)
 const turnstileSiteKey = ref<string>('')
 const siteName = ref<string>('Sub2API')
@@ -294,22 +295,25 @@ const errors = reactive({
 // ==================== Lifecycle ====================
 
 onMounted(async () => {
-  // Read promo code from URL parameter
-  const promoParam = route.query.promo as string
-  if (promoParam) {
-    formData.promo_code = promoParam
-    // Validate the promo code from URL
-    await validatePromoCodeDebounced(promoParam)
-  }
-
   try {
     const settings = await getPublicSettings()
     registrationEnabled.value = settings.registration_enabled
     emailVerifyEnabled.value = settings.email_verify_enabled
+    promoCodeEnabled.value = settings.promo_code_enabled
     turnstileEnabled.value = settings.turnstile_enabled
     turnstileSiteKey.value = settings.turnstile_site_key || ''
     siteName.value = settings.site_name || 'Sub2API'
     linuxdoOAuthEnabled.value = settings.linuxdo_oauth_enabled
+
+    // Read promo code from URL parameter only if promo code is enabled
+    if (promoCodeEnabled.value) {
+      const promoParam = route.query.promo as string
+      if (promoParam) {
+        formData.promo_code = promoParam
+        // Validate the promo code from URL
+        await validatePromoCodeDebounced(promoParam)
+      }
+    }
   } catch (error) {
     console.error('Failed to load public settings:', error)
   } finally {