add service worker for offline access

lib/public/service-worker.js

@@ -0,0 +1,19 @@
+self.addEventListener('install', (event) => {
+  event.waitUntil(
+    caches.open('fb-to-ical').then((cache) => {
+      return cache.addAll([
+        '/',
+        '/favicon.ico',
+      ])
+    })
+  )
+})
+
+self.addEventListener('fetch', (event) => {
+  event.respondWith(
+    caches.match(event.request)
+      .then((response) => {
+        return response || fetch(event.request)
+      })
+  )
+})