main.go: open test window on bare 'drover' (no subcommand)

Without this, double-clicking drover.exe printed CLI help to stdout —
fine in a terminal but invisible to a user who just clicked an icon.
Adding a RunE on the root command opens the smoke-test message box
when no subcommand was given. CLI flags (--version, --help) and
explicit subcommands keep their old behaviour.

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

cmd/drover/main.go

@@ -39,6 +39,13 @@ func newRootCmd() *cobra.Command {
 		Version:       fmt.Sprintf("%s (commit %s, built %s)", Version, Commit, BuildDate),
 		SilenceUsage:  true,
 		SilenceErrors: false,
+		// No subcommand and no flags = end-user double-clicked the exe;
+		// open the smoke-test window instead of dumping CLI help to a
+		// console they didn't ask for.
+		RunE: func(cmd *cobra.Command, args []string) error {
+			showTestWindow()
+			return nil
+		},
 	}
 
 	// Custom version template: "drover-go vX.Y.Z (commit abc1234, built 2026-05-01)".