fix(notify): add balance/quota notify flags to PublicSettings DTO and handler

The service layer correctly populated BalanceLowNotifyEnabled and
AccountQuotaNotifyEnabled in PublicSettings, but the handler-to-DTO
mapping was missing. Users could not see the balance notify card because
the public settings API never returned these flags.

backend/internal/handler/dto/settings.go

@@ -191,6 +191,8 @@ type PublicSettings struct {
 	BackendModeEnabled               bool             `json:"backend_mode_enabled"`
 	PaymentEnabled                   bool             `json:"payment_enabled"`
 	Version                          string           `json:"version"`
+	BalanceLowNotifyEnabled          bool             `json:"balance_low_notify_enabled"`
+	AccountQuotaNotifyEnabled        bool             `json:"account_quota_notify_enabled"`
 }
 
 // OverloadCooldownSettings 529过载冷却配置 DTO