feat(oauth2): add dashboard_host setting for callback URL

Decouple OAuth2 callback host from agent install host. When dashboard_host
is empty, the request Host is passed through; otherwise non-reserved hosts
are pinned to dashboard_host. Added to reserved-host allowlist for NAT.

cmd/dashboard/controller/setting.go

@@ -99,6 +99,7 @@ func updateConfig(c *gin.Context) (any, error) {
 	singleton.Conf.EnablePlainIPInNotification = sf.EnablePlainIPInNotification
 	singleton.Conf.Cover = sf.Cover
 	singleton.Conf.InstallHost = sf.InstallHost
+	singleton.Conf.DashboardHost = sf.DashboardHost
 	singleton.Conf.ReservedHosts = sf.ReservedHosts
 	singleton.Conf.IgnoredIPNotification = sf.IgnoredIPNotification
 	singleton.Conf.IPChangeNotificationGroupID = sf.IPChangeNotificationGroupID