feat: tunnel host and port in the panel settings, ufw kept in sync

The direct tunnel port (8446 by default) and the host agents dial for
it can be set in Settings. The server moves its listeners at once and
agents pick the address up from the panel: on connect and with every
poll, unless TUNNEL_ADDR / TUNNEL_PORT pin it locally.

ufw: run with a C locale (the output is translated), do not adopt rules
that existed before the panel, skip loopback-only listeners, re-open
what saved forwards need on start and when the integration is turned
on, move the shared tunnel rule with the port and drop it when no
direct forward is left. The last ufw error is shown in Settings.

The forwards page shows the real tunnel and panel ports.

internal/server/config.go

@@ -3,6 +3,7 @@ package server
 import (
 	"crypto/rand"
 	"encoding/hex"
+	"strconv"
 
 	lpra "github.com/x-grk/longpolling-remote-access"
 )
@@ -14,6 +15,8 @@ const (
 	cfgSetupDone    = "setup_done"
 	cfgInstallCode  = "install_code"
 	cfgUfwEnabled   = "ufw_enabled"
+	cfgTunnelHost   = "tunnel_host"
+	cfgTunnelPort   = "tunnel_port"
 )
 
 // DefaultHostAddress is the public URL agents dial by default. It is baked into
@@ -88,6 +91,24 @@ func (db *DB) SetUfwEnabled(on bool) error {
 	return db.SetConfig(cfgUfwEnabled, "0")
 }
 
+// TunnelHost is the host agents dial for the direct tunnel. Empty means the
+// host of the URL they already reach the panel on.
+func (db *DB) TunnelHost() string {
+	v, _ := db.GetConfig(cfgTunnelHost)
+	return v
+}
+
+// TunnelPort is the direct-tunnel port set in Settings, or 0 when it is left
+// at the port of the server's -tunnel address.
+func (db *DB) TunnelPort() int {
+	v, _ := db.GetConfig(cfgTunnelPort)
+	n, _ := strconv.Atoi(v)
+	if n < 1 || n > 65535 {
+		return 0
+	}
+	return n
+}
+
 func (db *DB) SetupDone() bool {
 	v, _ := db.GetConfig(cfgSetupDone)
 	return v == "1"