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/dto/settings.go

@@ -193,6 +193,7 @@ type PublicSettings struct {
 	Version                          string           `json:"version"`
 	BalanceLowNotifyEnabled          bool             `json:"balance_low_notify_enabled"`
 	AccountQuotaNotifyEnabled        bool             `json:"account_quota_notify_enabled"`
+	BalanceLowNotifyThreshold        float64          `json:"balance_low_notify_threshold"`
 }
 
 // OverloadCooldownSettings 529过载冷却配置 DTO