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/console_windows.go

@@ -0,0 +1,46 @@
+//go:build windows
+
+package main
+
+import (
+	"os"
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+// attachToParentConsole reconnects stdin/stdout/stderr to the console of the
+// process that launched us, if any. It is a no-op when the binary was started
+// from Explorer (double-click) — there is no parent console to attach to.
+//
+// Why we need this: the binary is built with -H=windowsgui, which means
+// Windows treats it as a GUI application and does NOT allocate a console
+// when it starts. That gives a clean double-click experience (no flashing
+// console window). But it also disconnects fmt.Println output when the
+// user runs us from cmd.exe / PowerShell. Calling AttachConsole here
+// re-attaches us to the parent's console so CLI output works as expected.
+func attachToParentConsole() {
+	const ATTACH_PARENT_PROCESS = ^uint32(0) // (DWORD)-1
+
+	kernel32 := windows.NewLazySystemDLL("kernel32.dll")
+	attachConsole := kernel32.NewProc("AttachConsole")
+
+	r, _, _ := attachConsole.Call(uintptr(ATTACH_PARENT_PROCESS))
+	if r == 0 {
+		// AttachConsole failed — most likely there is no parent console
+		// (we were started from Explorer). That's fine: GUI mode is the
+		// default UX and stdout/stderr just go nowhere.
+		return
+	}
+
+	// Re-bind os.Stdout / os.Stderr to the freshly-attached console.
+	if h, err := syscall.GetStdHandle(syscall.STD_OUTPUT_HANDLE); err == nil && h != 0 {
+		os.Stdout = os.NewFile(uintptr(h), "stdout")
+	}
+	if h, err := syscall.GetStdHandle(syscall.STD_ERROR_HANDLE); err == nil && h != 0 {
+		os.Stderr = os.NewFile(uintptr(h), "stderr")
+	}
+	if h, err := syscall.GetStdHandle(syscall.STD_INPUT_HANDLE); err == nil && h != 0 {
+		os.Stdin = os.NewFile(uintptr(h), "stdin")
+	}
+}