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)',

frontend/src/i18n/locales/zh.ts

@@ -598,6 +598,7 @@ export default {
         title: '订阅设置',
         type: '计费类型',
         typeHint: '标准计费从用户余额扣除。订阅模式使用配额限制。',
+        typeNotEditable: '分组创建后无法修改计费类型。',
         standard: '标准（余额）',
         subscription: '订阅（配额）',
         dailyLimit: '每日限额（USD）',