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/en.ts

@@ -3555,7 +3555,15 @@ export default {
         defaultBalance: 'Default Balance',
         defaultBalanceHint: 'Initial balance for new users',
         defaultConcurrency: 'Default Concurrency',
-        defaultConcurrencyHint: 'Maximum concurrent requests for new users'
+        defaultConcurrencyHint: 'Maximum concurrent requests for new users',
+        defaultSubscriptions: 'Default Subscriptions',
+        defaultSubscriptionsHint: 'Auto-assign these subscriptions when a new user is created or registered',
+        addDefaultSubscription: 'Add Default Subscription',
+        defaultSubscriptionsEmpty: 'No default subscriptions configured.',
+        defaultSubscriptionsDuplicate:
+          'Duplicate subscription group: {groupId}. Each group can only appear once.',
+        subscriptionGroup: 'Subscription Group',
+        subscriptionValidityDays: 'Validity (days)'
       },
       claudeCode: {
         title: 'Claude Code Settings',