Add decrement button to counter example

Address review feedback to include a decrement button that
demonstrates reactive updates in both directions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/counter/main.go

@@ -25,6 +25,11 @@ func main() {
 		count.Set(count.Get() + 1)
 	}, ui.TextFromString("Increment")))
 
+	// Button that decrements the count on click
+	view.Child(ui.Button(func() {
+		count.Set(count.Get() - 1)
+	}, ui.TextFromString("Decrement")))
+
 	ui.NewApp(view)
 
 	select {}