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/en.ts

@@ -911,17 +911,19 @@ export default {
       thresholdHint: 'Leave empty to use system default',
       thresholdPlaceholder: 'Enter amount',
       systemDefault: 'System Default',
-      extraEmails: 'Extra Notification Emails',
+      extraEmails: 'Notification Emails',
+      primaryEmail: 'Primary',
       noExtraEmails: 'No extra notification emails',
       enterEmail: 'Enter email address',
       addEmail: 'Add Email',
       emailPlaceholder: 'Enter email address',
       sendCode: 'Send Code',
+      resend: 'Resend',
       codeSent: 'Verification code sent',
       codeSentTo: 'Code sent to {email}',
       enterCode: 'Enter verification code',
       codePlaceholder: '6-digit code',
-      verify: 'Verify & Add',
+      verify: 'Verify',
       emailAdded: 'Email added',
       emailRemoved: 'Email removed',
       verifySuccess: 'Email added successfully',