Fix config hot-reload for serverIP

- Add getServerIP() method to dynamically read Gateway from config
- Update sendOffer, sendACK, sendNAK to use getServerIP()
- This ensures gateway IP updates when config changes via web UI

internal/dhcp/server.go

@@ -76,6 +76,12 @@ func (s *Server) getConfig() *config.DHCPConfig {
 	return s.config
 }
 
+// getServerIP returns the server IP for DHCP options
+func (s *Server) getServerIP() net.IP {
+	cfg := s.getConfig()
+	return net.ParseIP(cfg.Gateway).To4()
+}
+
 func (s *Server) Start() error {
 	if !s.config.Enabled {
 		return nil
@@ -383,7 +389,7 @@ func (s *Server) sendOffer(data []byte, clientMAC, offeredIP string, remoteAddr
 	response = appendOption(response, OptionLeaseTime, leaseTime)
 
 	// Add server identifier
-	response = appendOption(response, OptionServerIdentifier, []byte(s.serverIP.To4()))
+	response = appendOption(response, OptionServerIdentifier, []byte(s.getServerIP().To4()))
 
 	// Add end option
 	response = append(response, OptionEnd)
@@ -412,7 +418,7 @@ func (s *Server) sendACK(data []byte, clientMAC, ip string, remoteAddr *net.UDPA
 	binary.BigEndian.PutUint32(leaseTime, uint32(s.getConfig().LeaseTime))
 	response = appendOption(response, OptionLeaseTime, leaseTime)
 
-	response = appendOption(response, OptionServerIdentifier, []byte(s.serverIP.To4()))
+	response = appendOption(response, OptionServerIdentifier, []byte(s.getServerIP().To4()))
 	response = append(response, OptionEnd)
 
 	targetAddr := s.getResponseAddr(data, remoteAddr)
@@ -421,7 +427,7 @@ func (s *Server) sendACK(data []byte, clientMAC, ip string, remoteAddr *net.UDPA
 
 func (s *Server) sendNAK(data []byte, remoteAddr *net.UDPAddr) {
 	response := buildDHCPMessage(MsgNAK, data, "0.0.0.0", s.config)
-	response = appendOption(response, OptionServerIdentifier, []byte(s.serverIP.To4()))
+	response = appendOption(response, OptionServerIdentifier, []byte(s.getServerIP().To4()))
 	response = append(response, OptionEnd)
 
 	// NAK must always be broadcast