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/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",