Yeah your code does appear correct. So it seems your problem isn’t signature verification at all but just getting the server to respond to any requests coming from your app blocks.
I notice you use the fetch method to automatically construct the url from the base url of the page, but in my code I manually constructed the url myself. Maybe you could try that? Like here’s how I generated my app base url, which you can try using in your fetch method (you’d just add on the “/proxy/test” to the the URL returned from makeAppBaseUrl())
Sorry I won’t have much more time to help you, but I’m sure you can get it to work! It might just take some more debugging to figure out where the problem point is.