Register a service worker

To make installing as a PWA possible. Fixes #1181.

src/browser/serviceWorker.ts

@@ -0,0 +1,24 @@
+/* eslint-disable @typescript-eslint/no-explicit-any */
+
+self.addEventListener("install", () => {
+  console.log("[Service Worker] install")
+})
+
+self.addEventListener("activate", (event: any) => {
+  event.waitUntil((self as any).clients.claim())
+})
+
+self.addEventListener("fetch", (event: any) => {
+  if (!navigator.onLine) {
+    event.respondWith(
+      new Promise((resolve) => {
+        resolve(
+          new Response("OFFLINE", {
+            status: 200,
+            statusText: "OK",
+          }),
+        )
+      }),
+    )
+  }
+})