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/theme.go

@@ -0,0 +1,124 @@
+//go:build kanholec_gui
+
+package gui
+
+import (
+	"image/color"
+
+	"fyne.io/fyne/v2"
+	"fyne.io/fyne/v2/theme"
+)
+
+var (
+	colorPrimary     = color.NRGBA{R: 99, G: 102, B: 241, A: 255}
+	colorPrimaryDark = color.NRGBA{R: 79, G: 70, B: 229, A: 255}
+	colorSecondary   = color.NRGBA{R: 236, G: 72, B: 153, A: 255}
+	colorBackground  = color.NRGBA{R: 17, G: 24, B: 39, A: 255}
+	colorSurface     = color.NRGBA{R: 31, G: 41, B: 55, A: 255}
+	colorForeground  = color.NRGBA{R: 243, G: 244, B: 246, A: 255}
+	colorButton      = color.NRGBA{R: 55, G: 65, B: 81, A: 255}
+	colorHover       = color.NRGBA{R: 75, G: 85, B: 99, A: 255}
+	colorDisabled    = color.NRGBA{R: 107, G: 114, B: 128, A: 255}
+	colorInputBorder = color.NRGBA{R: 75, G: 85, B: 99, A: 255}
+	colorPlaceholder = color.NRGBA{R: 156, G: 163, B: 175, A: 255}
+	colorSuccess     = color.NRGBA{R: 34, G: 197, B: 94, A: 255}
+	colorWarning     = color.NRGBA{R: 251, G: 146, B: 60, A: 255}
+	colorError       = color.NRGBA{R: 239, G: 68, B: 68, A: 255}
+	colorSeparator   = color.NRGBA{R: 55, G: 65, B: 81, A: 255}
+	colorAccent      = color.NRGBA{R: 139, G: 92, B: 246, A: 255}
+)
+
+type kanholeTheme struct{}
+
+var _ fyne.Theme = (*kanholeTheme)(nil)
+
+func (t *kanholeTheme) Color(name fyne.ThemeColorName, variant fyne.ThemeVariant) color.Color {
+	switch name {
+	case theme.ColorNameBackground:
+		return colorBackground
+	case theme.ColorNameForeground, theme.ColorNameForegroundOnPrimary:
+		return colorForeground
+	case theme.ColorNamePrimary:
+		return colorPrimary
+	case theme.ColorNameButton:
+		return colorButton
+	case theme.ColorNameHover:
+		return colorHover
+	case theme.ColorNameDisabled:
+		return colorDisabled
+	case theme.ColorNameDisabledButton:
+		return color.NRGBA{R: 45, G: 55, B: 72, A: 255}
+	case theme.ColorNameInputBorder:
+		return colorInputBorder
+	case theme.ColorNamePlaceHolder:
+		return colorPlaceholder
+	case theme.ColorNameScrollBar:
+		return color.NRGBA{R: 107, G: 114, B: 128, A: 180}
+	case theme.ColorNameShadow:
+		return color.NRGBA{R: 0, G: 0, B: 0, A: 100}
+	case theme.ColorNameSeparator:
+		return colorSeparator
+	case theme.ColorNameFocus:
+		return color.NRGBA{R: 99, G: 102, B: 241, A: 140}
+	case theme.ColorNameSelection:
+		return color.NRGBA{R: 99, G: 102, B: 241, A: 80}
+	case theme.ColorNameInputBackground:
+		return colorSurface
+	case theme.ColorNameHeaderBackground:
+		return color.NRGBA{R: 17, G: 24, B: 39, A: 255}
+	default:
+		return theme.DefaultTheme().Color(name, theme.VariantDark)
+	}
+}
+
+func (t *kanholeTheme) Font(style fyne.TextStyle) fyne.Resource {
+	return theme.DefaultTheme().Font(style)
+}
+
+func (t *kanholeTheme) Icon(name fyne.ThemeIconName) fyne.Resource {
+	return theme.DefaultTheme().Icon(name)
+}
+
+func (t *kanholeTheme) Size(name fyne.ThemeSizeName) float32 {
+	switch name {
+	case theme.SizeNamePadding:
+		return 10
+	case theme.SizeNameInnerPadding:
+		return 8
+	case theme.SizeNameLineSpacing:
+		return 32
+	case theme.SizeNameScrollBar:
+		return 10
+	case theme.SizeNameScrollBarSmall:
+		return 5
+	case theme.SizeNameText:
+		return 14
+	case theme.SizeNameHeadingText:
+		return 28
+	case theme.SizeNameSubHeadingText:
+		return 18
+	case theme.SizeNameCaptionText:
+		return 12
+	case theme.SizeNameInputBorder:
+		return 2
+	case theme.SizeNameInputRadius:
+		return 10
+	case theme.SizeNameSelectionRadius:
+		return 8
+	default:
+		return theme.DefaultTheme().Size(name)
+	}
+}
+
+func StatusColor(s string) color.Color {
+	switch s {
+	case "running", "connected", "ok", "active":
+		return colorSuccess
+	case "warning", "reconnecting":
+		return colorWarning
+	case "error", "stopped", "failed", "disconnected":
+		return colorError
+	default:
+		return colorPlaceholder
+	}
+}