fix(notify): remove percentage threshold from balance notification

Balance low notification only supports fixed USD amount threshold.
Percentage threshold is a quota concept, not applicable to balance.
Reverted threshold_type from admin settings, user profile, and all
backend/frontend layers. DB fields (balance_notify_threshold_type,
total_recharged) retained for potential future quota use.

frontend/src/i18n/locales/zh.ts

@@ -4797,12 +4797,8 @@ export default {
         title: '余额不足提醒',
         description: '当用户余额低于阈值时发送邮件提醒',
         enabled: '启用余额不足提醒',
-        thresholdType: '阈值类型',
-        typeFixed: '固定金额',
-        typePercentage: '充值百分比',
-        threshold: '提醒阈值',
+        threshold: '默认提醒阈值',
         thresholdHint: '用户未自定义时使用此值',
-        percentageHint: '当余额低于累计充值额的此百分比时提醒',
         thresholdPlaceholder: '输入金额',
       },
       quotaNotify: {