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)

packaging/windows/kanholec.wixproj

@@ -0,0 +1,13 @@
+<Project Sdk="WixToolset.Sdk/7.0.0">
+  <PropertyGroup>
+    <InstallerPlatform>x64</InstallerPlatform>
+    <OutputName>kanholec-0.69.0</OutputName>
+    <OutputType>Package</OutputType>
+    <DefineConstants>Version=0.69.0</DefineConstants>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="WixToolset.UI.wixext" Version="7.0.0" />
+    <PackageReference Include="WixToolset.Util.wixext" Version="7.0.0" />
+  </ItemGroup>
+</Project>