feat(notify): improve balance notify card UX

- Show system default threshold as placeholder in custom threshold input
- Display user's primary email with "Primary" badge
- Support adding multiple pending emails before verification
- Each pending email has independent send/verify/resend flow
- Expose balance_low_notify_threshold in PublicSettings API
- Clean up timers on unmount to prevent leaks

frontend/src/i18n/locales/zh.ts

@@ -915,17 +915,19 @@ export default {
       thresholdHint: '留空使用系统默认值',
       thresholdPlaceholder: '输入金额',
       systemDefault: '系统默认值',
-      extraEmails: '额外通知邮箱',
+      extraEmails: '通知邮箱',
+      primaryEmail: '主邮箱',
       noExtraEmails: '暂无额外通知邮箱',
       enterEmail: '输入邮箱地址',
       addEmail: '添加邮箱',
       emailPlaceholder: '输入邮箱地址',
       sendCode: '发送验证码',
+      resend: '重发',
       codeSent: '验证码已发送',
       codeSentTo: '验证码已发送到 {email}',
       enterCode: '输入验证码',
       codePlaceholder: '6位验证码',
-      verify: '确认添加',
+      verify: '验证',
       emailAdded: '邮箱已添加',
       emailRemoved: '邮箱已移除',
       verifySuccess: '邮箱添加成功',