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)

cmd/kanholec/main.go

@@ -1,3 +1,5 @@
+//go:build !kanholec_gui
+
 // Copyright 2016 fatedier, fatedier@gmail.com
 //
 // Licensed under the Apache License, Version 2.0 (the "License");