fix(frontend): 优化分组表单中订阅模式的字段显示逻辑

- 订阅模式下隐藏 Exclusive 字段并默认为开启状态
- 编辑分组时禁用计费类型字段，防止修改
- 移除编辑表单中无用的 subscription_type watch

frontend/src/i18n/locales/en.ts

@@ -543,6 +543,7 @@ export default {
         title: 'Subscription Settings',
         type: 'Billing Type',
         typeHint: 'Standard billing deducts from user balance. Subscription mode uses quota limits instead.',
+        typeNotEditable: 'Billing type cannot be changed after group creation.',
         standard: 'Standard (Balance)',
         subscription: 'Subscription (Quota)',
         dailyLimit: 'Daily Limit (USD)',