add service worker and progressive web app and icons

src/index.js

@@ -3,6 +3,7 @@ import ReactDOM from 'react-dom/client';
 import App from './App';
 import { HelmetProvider } from 'react-helmet-async';
 import { HashRouter } from 'react-router-dom';
+import * as serviceWorkerRegistration from './serviceWorkerRegistration';
 
 import PlebbitJs from '@plebbit/plebbit-js';
 // inject native functions into renderer
@@ -20,4 +21,7 @@ root.render(
       </HashRouter>
     </HelmetProvider>
   </React.StrictMode>
-);
+);
+
+// set up PWA https://cra.link/PWA
+serviceWorkerRegistration.register();