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

backend/internal/handler/setting_handler.go

@@ -63,5 +63,6 @@ func (h *SettingHandler) GetPublicSettings(c *gin.Context) {
 		Version:                          h.version,
 		BalanceLowNotifyEnabled:          settings.BalanceLowNotifyEnabled,
 		AccountQuotaNotifyEnabled:        settings.AccountQuotaNotifyEnabled,
+		BalanceLowNotifyThreshold:        settings.BalanceLowNotifyThreshold,
 	})
 }