Implement PWA features: add service worker, manifest, and offline page; enhance footer and header navigation

_layouts/default.html

@@ -9,5 +9,12 @@
     {{ content }}
   </main>
   {% include footer.html %}
+  <script>
+    if ('serviceWorker' in navigator) {
+      navigator.serviceWorker.register("{{ /assets/js/service-worker.js | relative_url }}")
+        .then(() => console.log("✅ Service Worker registriert"))
+        .catch(error => console.log("❌ Service Worker Fehler:", error));
+    }
+  </script>
 </body>
 </html>