[client] Add RDP token passthrough for passwordless Windows Remote Desktop

Implement sideband authorization and credential provider architecture for
passwordless RDP access to Windows peers via NetBird.

Go components:
- Sideband RDP auth server (TCP on WG interface, port 3390/22023)
- Pending session store with TTL expiry and replay protection
- Named pipe IPC server (\\.\pipe\netbird-rdp-auth) for credential provider
- Sideband client for connecting peer to request authorization
- CLI command `netbird rdp [user@]host` with JWT auth flow
- Engine integration with DNAT port redirection

Rust credential provider DLL (client/rdp/credprov/):
- COM DLL implementing ICredentialProvider + ICredentialProviderCredential
- Loaded by Windows LogonUI.exe at the RDP login screen
- Queries NetBird agent via named pipe for pending sessions
- Performs S4U logon (LsaLogonUser) for passwordless Windows token creation
- Self-registration via regsvr32 (DllRegisterServer/DllUnregisterServer)

https://claude.ai/code/session_01C38bCDyYzLgxYLVwJkcUng

client/internal/engine.go

@@ -197,6 +197,7 @@ type Engine struct {
 	networkMonitor *networkmonitor.NetworkMonitor
 
 	sshServer sshServer
+	rdpServer rdpServer
 
 	statusRecorder *peer.Status
 

client/internal/engine_rdp.go

@@ -0,0 +1,123 @@
+package internal
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/netip"
+
+	log "github.com/sirupsen/logrus"
+
+	firewallManager "github.com/netbirdio/netbird/client/firewall/manager"
+	nftypes "github.com/netbirdio/netbird/client/internal/netflow/types"
+	rdpserver "github.com/netbirdio/netbird/client/rdp/server"
+)
+
+type rdpServer interface {
+	Start(ctx context.Context, addr netip.AddrPort) error
+	Stop() error
+	GetPendingStore() *rdpserver.PendingStore
+}
+
+func (e *Engine) setupRDPPortRedirection() error {
+	if e.firewall == nil || e.wgInterface == nil {
+		return nil
+	}
+
+	localAddr := e.wgInterface.Address().IP
+	if !localAddr.IsValid() {
+		return errors.New("invalid local NetBird address")
+	}
+
+	if err := e.firewall.AddInboundDNAT(localAddr, firewallManager.ProtocolTCP, rdpserver.DefaultRDPAuthPort, rdpserver.InternalRDPAuthPort); err != nil {
+		return fmt.Errorf("add RDP auth port redirection: %w", err)
+	}
+	log.Infof("RDP auth port redirection enabled: %s:%d -> %s:%d",
+		localAddr, rdpserver.DefaultRDPAuthPort, localAddr, rdpserver.InternalRDPAuthPort)
+
+	return nil
+}
+
+func (e *Engine) cleanupRDPPortRedirection() error {
+	if e.firewall == nil || e.wgInterface == nil {
+		return nil
+	}
+
+	localAddr := e.wgInterface.Address().IP
+	if !localAddr.IsValid() {
+		return errors.New("invalid local NetBird address")
+	}
+
+	if err := e.firewall.RemoveInboundDNAT(localAddr, firewallManager.ProtocolTCP, rdpserver.DefaultRDPAuthPort, rdpserver.InternalRDPAuthPort); err != nil {
+		return fmt.Errorf("remove RDP auth port redirection: %w", err)
+	}
+	log.Debugf("RDP auth port redirection removed: %s:%d -> %s:%d",
+		localAddr, rdpserver.DefaultRDPAuthPort, localAddr, rdpserver.InternalRDPAuthPort)
+
+	return nil
+}
+
+func (e *Engine) startRDPServer() error {
+	if e.wgInterface == nil {
+		return errors.New("wg interface not initialized")
+	}
+
+	wgAddr := e.wgInterface.Address()
+
+	cfg := &rdpserver.Config{
+		NetworkAddr: wgAddr.Network,
+	}
+
+	server := rdpserver.New(cfg)
+
+	netbirdIP := wgAddr.IP
+	listenAddr := netip.AddrPortFrom(netbirdIP, rdpserver.InternalRDPAuthPort)
+
+	if err := server.Start(e.ctx, listenAddr); err != nil {
+		return fmt.Errorf("start RDP auth server: %w", err)
+	}
+
+	e.rdpServer = server
+
+	if netstackNet := e.wgInterface.GetNet(); netstackNet != nil {
+		if registrar, ok := e.firewall.(interface {
+			RegisterNetstackService(protocol nftypes.Protocol, port uint16)
+		}); ok {
+			registrar.RegisterNetstackService(nftypes.TCP, rdpserver.InternalRDPAuthPort)
+			log.Debugf("registered RDP auth service with netstack for TCP:%d", rdpserver.InternalRDPAuthPort)
+		}
+	}
+
+	if err := e.setupRDPPortRedirection(); err != nil {
+		log.Warnf("failed to setup RDP auth port redirection: %v", err)
+	}
+
+	return nil
+}
+
+func (e *Engine) stopRDPServer() error {
+	if e.rdpServer == nil {
+		return nil
+	}
+
+	if err := e.cleanupRDPPortRedirection(); err != nil {
+		log.Warnf("failed to cleanup RDP auth port redirection: %v", err)
+	}
+
+	if netstackNet := e.wgInterface.GetNet(); netstackNet != nil {
+		if registrar, ok := e.firewall.(interface {
+			UnregisterNetstackService(protocol nftypes.Protocol, port uint16)
+		}); ok {
+			registrar.UnregisterNetstackService(nftypes.TCP, rdpserver.InternalRDPAuthPort)
+			log.Debugf("unregistered RDP auth service from netstack for TCP:%d", rdpserver.InternalRDPAuthPort)
+		}
+	}
+
+	log.Info("stopping RDP auth server")
+	err := e.rdpServer.Stop()
+	e.rdpServer = nil
+	if err != nil {
+		return fmt.Errorf("stop: %w", err)
+	}
+	return nil
+}