Fix dnsmasq crash by creating tftp-root dir and using subnet config

dnsmasq exited with status 3 because the tftp-root directory didn't
exist at startup. Also replaced hardcoded 192.168.1.0 in dhcp-range
with the configured subnet value.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/pxe/supervisor.go

@@ -41,6 +41,9 @@ func (s *Supervisor) Start(ctx context.Context, hosts []model.Host) error {
 	if err := os.MkdirAll(s.cfg.RuntimeDir, 0o755); err != nil {
 		return fmt.Errorf("pxe: create runtime dir: %w", err)
 	}
+	if err := os.MkdirAll(s.cfg.TFTPRoot, 0o755); err != nil {
+		return fmt.Errorf("pxe: create tftp root: %w", err)
+	}
 	if err := s.writeConfig(hosts); err != nil {
 		return err
 	}
@@ -82,8 +85,14 @@ func (s *Supervisor) writeConfig(hosts []model.Host) error {
 		return fmt.Errorf("pxe: write dhcp-hostsfile: %w", err)
 	}
 
+	subnet := s.cfg.Subnet
+	if i := strings.Index(subnet, "/"); i != -1 {
+		subnet = subnet[:i]
+	}
+
 	conf := dnsmasqConf{
 		Interface:  s.cfg.Interface,
+		Subnet:     subnet,
 		TFTPRoot:   s.cfg.TFTPRoot,
 		HostsFile:  hostsPath,
 		PublicURL:  s.cfg.PublicURL,
@@ -126,6 +135,7 @@ func (s *Supervisor) startProcess(ctx context.Context) error {
 
 type dnsmasqConf struct {
 	Interface  string
+	Subnet     string
 	TFTPRoot   string
 	HostsFile  string
 	PublicURL  string
@@ -137,7 +147,7 @@ port=0
 interface={{.Interface}}
 bind-interfaces
 
-dhcp-range=tag:known,192.168.1.0,proxy
+dhcp-range=tag:known,{{.Subnet}},proxy
 dhcp-hostsfile={{.HostsFile}}
 dhcp-ignore=tag:!known