Emit FwMark in generated wg.conf when set

The Firewall Mark from global settings was never written to the
WireGuard config. Render it in the [Interface] section when present.

Refs: ngoduykhanh/wireguard-ui#437
Made-with: Cursor

Author: gaoyifan

templates/wg.conf

@@ -8,6 +8,7 @@ Address = {{$first :=true}}{{range .serverConfig.Interface.Addresses }}{{if $fir
 ListenPort = {{ .serverConfig.Interface.ListenPort }}
 PrivateKey = {{ .serverConfig.KeyPair.PrivateKey }}
 {{if .globalSettings.MTU}}MTU = {{ .globalSettings.MTU }}{{end}}
+{{if .globalSettings.FirewallMark }}FwMark = {{ .globalSettings.FirewallMark }}{{end}}
 PostUp = {{ .serverConfig.Interface.PostUp }}
 PreDown = {{ .serverConfig.Interface.PreDown }}
 PostDown = {{ .serverConfig.Interface.PostDown }}