feat(notify): add platform/ID to quota alert email, add recharge URL to balance alert

- Quota alert email now shows account ID and platform
- Balance low email includes a "Top Up Now" button when recharge URL is configured
- New setting: balance_low_notify_recharge_url in admin settings

frontend/src/i18n/locales/en.ts

@@ -4649,6 +4649,9 @@ export default {
         threshold: 'Default Threshold',
         thresholdHint: 'Used when user has not set a custom value',
         thresholdPlaceholder: 'Enter amount',
+        rechargeUrl: 'Recharge Page URL',
+        rechargeUrlPlaceholder: 'https://example.com/payment',
+        rechargeUrlHint: 'A top-up button will appear in the email when set',
       },
       quotaNotify: {
         title: 'Account Quota Notification',

frontend/src/i18n/locales/zh.ts

@@ -4813,6 +4813,9 @@ export default {
         threshold: '默认提醒阈值',
         thresholdHint: '用户未自定义时使用此值',
         thresholdPlaceholder: '输入金额',
+        rechargeUrl: '充值页面 URL',
+        rechargeUrlPlaceholder: 'https://example.com/payment',
+        rechargeUrlHint: '设置后邮件中将包含充值链接按钮',
       },
       quotaNotify: {
         title: '账号限额通知',