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)
+}