Add 'drover gui' subcommand: native Win32 MessageBox smoke test

Shows a small message box with version metadata + build info. Used as
a fastest possible end-user smoke check after install: double-click
drover.exe in a portable bundle (or run 'drover gui' from cmd) and
see "OK — the binary launched and the Windows API is reachable" with
the right version on screen. No network calls, no admin needed.

Implementation: cgo-free, uses only golang.org/x/sys/windows (already
pulled in by the updater package). gui_other.go is a stub so the
package still compiles on Linux for the CI smoke build.

Locally verified: built with -X main.Version=test-local, ran
'drover.exe gui' on Windows 11, message box appeared with correct
version/commit/Go runtime info.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/drover/gui_other.go

@@ -0,0 +1,9 @@
+//go:build !windows
+
+package main
+
+import "fmt"
+
+func showTestWindow() {
+	fmt.Printf("Drover-Go v%s — test window unavailable on non-Windows builds\n", Version)
+}

cmd/drover/gui_windows.go

@@ -0,0 +1,50 @@
+//go:build windows
+
+package main
+
+import (
+	"fmt"
+	"runtime"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// showTestWindow displays a native Win32 MessageBox with build info.
+// The intent is to give end-users a visual smoke-test on first run:
+// double-click drover.exe (or run `drover gui`) and see that:
+//   1. the binary actually launches on Windows,
+//   2. the embedded version metadata is correct,
+//   3. the process can talk to user32.dll (i.e. the runtime is healthy).
+//
+// This is *not* the production GUI — that comes later via Wails. Here we
+// purposely use only stdlib + golang.org/x/sys/windows so this works
+// before any Wails/CGO machinery is wired up.
+func showTestWindow() {
+	user32 := windows.NewLazySystemDLL("user32.dll")
+	messageBox := user32.NewProc("MessageBoxW")
+
+	body := fmt.Sprintf(
+		"Drover-Go v%s\n\n"+
+			"Commit:   %s\n"+
+			"Build:    %s\n"+
+			"Go:       %s\n"+
+			"Arch:     %s/%s\n\n"+
+			"OK — the binary launched and the Windows API is reachable.",
+		Version, Commit, BuildDate, runtime.Version(), runtime.GOOS, runtime.GOARCH,
+	)
+	title := fmt.Sprintf("Drover-Go v%s — test window", Version)
+
+	bodyW, _ := windows.UTF16PtrFromString(body)
+	titleW, _ := windows.UTF16PtrFromString(title)
+
+	// MB_OK | MB_ICONINFORMATION | MB_SETFOREGROUND
+	const flags = 0x00000000 | 0x00000040 | 0x00010000
+
+	messageBox.Call(
+		0,
+		uintptr(unsafe.Pointer(bodyW)),
+		uintptr(unsafe.Pointer(titleW)),
+		flags,
+	)
+}

cmd/drover/main.go

@@ -49,10 +49,22 @@ func newRootCmd() *cobra.Command {
 	root.AddCommand(newCheckCmd())
 	root.AddCommand(newUpdateCmd())
 	root.AddCommand(newServiceCmd())
+	root.AddCommand(newGUICmd())
 
 	return root
 }
 
+func newGUICmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "gui",
+		Short: "Show a test window (smoke check that the binary launches on this machine)",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			showTestWindow()
+			return nil
+		},
+	}
+}
+
 func newCheckCmd() *cobra.Command {
 	return &cobra.Command{
 		Use:   "check",