add simple service worker

public/sw.js

···
+// This is the "Offline copy of pages" service worker
+
+const CACHE = "pwabuilder-offline";
+
+importScripts('https://storage.googleapis.com/workbox-cdn/releases/5.1.2/workbox-sw.js');
+
+self.addEventListener("message", (event) => {
+  if (event.data && event.data.type === "SKIP_WAITING") {
+    self.skipWaiting();
+  }
+});
+
+workbox.routing.registerRoute(
+  new RegExp('/*'),
+  new workbox.strategies.StaleWhileRevalidate({
+    cacheName: CACHE
+  })
+);

src/layouts/Base.astro

···
     <meta name="generator" content={Astro.generator} />
     <title>{title}</title>
 
+    <script is:inline>
+      if (typeof navigator.serviceWorker !== 'undefined') {
+        navigator.serviceWorker.register('sw.js')
+      }
+    </script>
+
     <style is:global>
       /* instantly overwrite reset */
       @layer reset {