feat(gui): redesign kanholec GUI with modern interface and WiX v7 installer

- Complete GUI overhaul with Fyne framework
  * Modern dark theme with indigo/pink accent colors
  * 4-step setup wizard (Welcome → Server → Auth → Finish)
  * Dashboard with proxy list, real-time logs, and config viewer
  * Windows Service manager (install/start/stop/uninstall)
  * System tray integration with quick controls

- WiX v7 MSI installer
  * Auto-registers as Windows Service (manual start)
  * Feature tree UI for optional components
  * Desktop shortcut and PATH options
  * Build script: packaging/windows/build-msi.ps1

- Build system updates
  * Added kanholec-windows-gui target (CGO required)
  * Added kanholec-windows-msi target
  * Separate main_gui.go entry point for GUI builds

GUI binary: bin/kanholec-windows-amd64.exe (31.4 MB)
MSI installer: bin/kanholec-0.69.0-amd64.msi (12.1 MB)

client/gui/service.go

@@ -0,0 +1,141 @@
+//go:build kanholec_gui
+
+package gui
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+)
+
+type ServiceManager struct {
+	binaryPath  string
+	serviceName string
+}
+
+func NewServiceManager() *ServiceManager {
+	exe, _ := os.Executable()
+	return &ServiceManager{
+		binaryPath:  exe,
+		serviceName: "kanholec",
+	}
+}
+
+func (s *ServiceManager) IsInstalled() bool {
+	if runtime.GOOS != "windows" {
+		return false
+	}
+	cmd := exec.Command("sc", "query", s.serviceName)
+	err := cmd.Run()
+	return err == nil
+}
+
+func (s *ServiceManager) Install(configPath string) error {
+	if runtime.GOOS != "windows" {
+		return fmt.Errorf("service installation is only supported on Windows")
+	}
+
+	if s.IsInstalled() {
+		return fmt.Errorf("service %q is already installed", s.serviceName)
+	}
+
+	args := []string{
+		"create", s.serviceName,
+		"binPath=", fmt.Sprintf(`"%s" -c "%s"`, s.binaryPath, configPath),
+		"DisplayName=", "kanholec - kanhole Client",
+		"start=", "auto",
+		"obj=", "LocalSystem",
+	}
+
+	cmd := exec.Command("sc", args...)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("failed to install service: %s: %w", string(out), err)
+	}
+
+	descArgs := []string{"description", s.serviceName, "kanhole reverse proxy client service"}
+	cmd = exec.Command("sc", descArgs...)
+	cmd.Run()
+
+	return nil
+}
+
+func (s *ServiceManager) Uninstall() error {
+	if runtime.GOOS != "windows" {
+		return fmt.Errorf("service management is only supported on Windows")
+	}
+
+	if !s.IsInstalled() {
+		return fmt.Errorf("service %q is not installed", s.serviceName)
+	}
+
+	s.Stop()
+
+	cmd := exec.Command("sc", "delete", s.serviceName)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("failed to uninstall service: %s: %w", string(out), err)
+	}
+	return nil
+}
+
+func (s *ServiceManager) Start() error {
+	if runtime.GOOS != "windows" {
+		return fmt.Errorf("service management is only supported on Windows")
+	}
+
+	cmd := exec.Command("sc", "start", s.serviceName)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("failed to start service: %s: %w", string(out), err)
+	}
+	return nil
+}
+
+func (s *ServiceManager) Stop() error {
+	if runtime.GOOS != "windows" {
+		return fmt.Errorf("service management is only supported on Windows")
+	}
+
+	cmd := exec.Command("sc", "stop", s.serviceName)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("failed to stop service: %s: %w", string(out), err)
+	}
+	return nil
+}
+
+func (s *ServiceManager) IsRunning() bool {
+	if runtime.GOOS != "windows" {
+		return false
+	}
+	cmd := exec.Command("sc", "query", s.serviceName)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return false
+	}
+	return contains(string(out), "RUNNING")
+}
+
+func (s *ServiceManager) GetConfigPath() string {
+	programData := os.Getenv("ProgramData")
+	if programData == "" {
+		programData = `C:\ProgramData`
+	}
+	return filepath.Join(programData, "kanholec", "kanholec.toml")
+}
+
+func contains(s, substr string) bool {
+	return len(s) >= len(substr) && searchString(s, substr)
+}
+
+func searchString(s, substr string) bool {
+	for i := 0; i <= len(s)-len(substr); i++ {
+		if s[i:i+len(substr)] == substr {
+			return true
+		}
+	}
+	return false
+}