[client] Fix the Windows tray deadlock on re-entrant window creation

The Wails systray runs the left-click handler synchronously inside the
tray window procedure, and creating a window on a running app pumps a
nested Win32 message loop while WebView2 initialises. ensureWindow held
the non-reentrant createMu across that creation, so the second button-up
of a double click re-entered ShowWindow from the pump and blocked the
main thread on its own lock. A goroutine holding createMu while the main
thread pumped, and the Open* dialogs holding mu across NewWithOptions,
Show, Hide and InvokeSync, exposed the same inversion.

WindowManager now serialises creation with a per-slot creating flag and
queues the callers' operations until the window exists, and no Wails call
runs while mu is held. The tray click and second-instance handlers call
ShowWindow off the message loop.

client/ui/main.go

@@ -100,7 +100,7 @@ func main() {
 	var tray *Tray
 	app := newApplication(func() {
 		if tray != nil {
-			tray.ShowWindow()
+			go tray.ShowWindow()
 		}
 	})