PWA: manifest.json, Service Worker, Meta-Tags und README-Hinweis hinzugefügt

static/service-worker.js

@@ -0,0 +1,20 @@
+const CACHE_NAME = 'datumsrechner-cache-v1';
+const urlsToCache = [
+  '/',
+  '/static/style.css',
+  '/favicon.ico',
+  '/favicon.png',
+  '/logo.svg',
+];
+self.addEventListener('install', event => {
+  event.waitUntil(
+    caches.open(CACHE_NAME)
+      .then(cache => cache.addAll(urlsToCache))
+  );
+});
+self.addEventListener('fetch', event => {
+  event.respondWith(
+    caches.match(event.request)
+      .then(response => response || fetch(event.request))
+  );
+});