fix(notify): use real-time balance for crossing detection and simplify email logic

- Fix cached balance causing threshold crossing to never trigger:
  read real-time balance from billingCacheService instead of stale
  API key auth snapshot
- Remove email="" placeholder concept; all emails are user-managed
- Only send notifications to verified && non-disabled emails
- Frontend: pre-fill user's email in add input when list is empty
- Remove FilterEnabledEmails/IsPrimaryDisabled helpers (no longer needed)

frontend/src/i18n/locales/zh.ts

@@ -936,6 +936,7 @@ export default {
       emailDuplicate: '该邮箱已存在',
       maxEmailsReached: '已达到通知邮箱数量上限',
       unverified: '未验证',
+      verified: '已验证',
     }
   },