Create sw.js

public/sw.js

@@ -0,0 +1,23 @@
+// /sw.js (simple test worker)
+self.addEventListener('install', event => {
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', event => {
+  event.waitUntil(self.clients.claim());
+});
+
+self.addEventListener('message', event => {
+  const data = event.data || {};
+  if (data?.type === 'PING') {
+    // prefer replying on the message port if provided
+    if (event.ports && event.ports[0]) {
+      event.ports[0].postMessage({type: 'PONG', ts: Date.now(), ok: true});
+    } else {
+      // fallback: broadcast to all clients
+      self.clients.matchAll().then(clients => {
+        clients.forEach(c => c.postMessage({type: 'PONG', ts: Date.now(), ok: true}));
+      });
+    }
+  }
+});