GUI subsystem: -H=windowsgui + AttachConsole, MB_TOPMOST on test window

drover.exe is now a GUI subsystem binary:
  - Double-click no longer flashes a console window — a clean
    smoke-test message box opens immediately.
  - When run from cmd / PowerShell, AttachConsole reattaches stdout
    and stderr to the parent terminal so '--version', 'check', etc.
    still print as expected.
  - MB_TOPMOST flag added to MessageBox so the window can't be
    obscured by other windows on launch (this was the actual cause
    of "I clicked but nothing happened" reports).

Verified locally: built with GOOS=windows GOARCH=amd64 -H=windowsgui;
running drover-gui.exe --version prints to PowerShell, drover-gui.exe
gui shows the message box on top of the active window.

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

cmd/drover/gui_windows.go

@@ -38,8 +38,10 @@ func showTestWindow() {
 	bodyW, _ := windows.UTF16PtrFromString(body)
 	titleW, _ := windows.UTF16PtrFromString(title)
 
-	// MB_OK | MB_ICONINFORMATION | MB_SETFOREGROUND
-	const flags = 0x00000000 | 0x00000040 | 0x00010000
+	// MB_OK | MB_ICONINFORMATION | MB_SETFOREGROUND | MB_TOPMOST
+	// MB_TOPMOST is essential — without it the message box can pop up
+	// behind other windows and the user thinks nothing happened.
+	const flags = 0x00000000 | 0x00000040 | 0x00010000 | 0x00040000
 
 	messageBox.Call(
 		0,