app/internal/window: remove context-menu (right-click) for wasm

The context-menu seems useless. The only action available, on the
context-menu is to "Save image": which gives a black image.

I think it's better to remove it. The right-click still work and still
provide `pointer.Event`.

Signed-off-by: Inkeliz <inkeliz@inkeliz.com>

app/internal/window/os_js.go

@@ -120,6 +120,10 @@ func (w *window) addEventListeners() {
 		w.draw(true)
 		return nil
 	})
+	w.addEventListener(w.window, "contextmenu", func(this js.Value, args []js.Value) interface{} {
+		args[0].Call("preventDefault")
+		return nil
+	})
 	w.addEventListener(w.cnv, "mousemove", func(this js.Value, args []js.Value) interface{} {
 		w.pointerEvent(pointer.Move, 0, 0, args[0])
 		return nil