feat(settings): add default subscriptions for new users

- add default subscriptions to admin settings

- auto-assign subscriptions on register and admin user creation

- add validation/tests and align settings UI with subscription selector patterns

frontend/src/i18n/locales/zh.ts

@@ -3725,7 +3725,14 @@ export default {
         defaultBalance: '默认余额',
         defaultBalanceHint: '新用户的初始余额',
         defaultConcurrency: '默认并发数',
-        defaultConcurrencyHint: '新用户的最大并发请求数'
+        defaultConcurrencyHint: '新用户的最大并发请求数',
+        defaultSubscriptions: '默认订阅列表',
+        defaultSubscriptionsHint: '新用户创建或注册时自动分配这些订阅',
+        addDefaultSubscription: '添加默认订阅',
+        defaultSubscriptionsEmpty: '未配置默认订阅。新用户不会自动获得订阅套餐。',
+        defaultSubscriptionsDuplicate: '默认订阅存在重复分组：{groupId}。每个分组只能出现一次。',
+        subscriptionGroup: '订阅分组',
+        subscriptionValidityDays: '有效期（天）'
       },
       claudeCode: {
         title: 'Claude Code 设置',