Add PWA support: implement service worker, cache assets, and add offline functionality

_layouts/default.html

@@ -11,7 +11,7 @@
   {% include footer.html %}
   <script>
     if ('serviceWorker' in navigator) {
-      navigator.serviceWorker.register("{{ /assets/js/service-worker.js | relative_url }}")
+      navigator.serviceWorker.register("{{ '/assets/js/service-worker.js' | relative_url }}")
         .then(() => console.log("✅ Service Worker registriert"))
         .catch(error => console.log("❌ Service Worker Fehler:", error));
     }