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/en.ts

@@ -4633,12 +4633,8 @@ export default {
         title: 'Balance Low Notification',
         description: 'Send email notification when user balance falls below threshold',
         enabled: 'Enable Balance Low Notification',
-        thresholdType: 'Threshold Type',
-        typeFixed: 'Fixed Amount',
-        typePercentage: 'Percentage of Recharged',
         threshold: 'Default Threshold',
         thresholdHint: 'Used when user has not set a custom value',
-        percentageHint: 'Notify when balance falls below this percentage of total recharged amount',
         thresholdPlaceholder: 'Enter amount',
       },
       quotaNotify: {