Offline support for service worker

cmd/fastgallery/assets/serviceWorker.js

@@ -7,7 +7,17 @@ self.addEventListener('activate', (event) => {
 });
 
 self.addEventListener('fetch', (event) => {
-    event.respondWith(
-        fetch(event.request)
-    );
+    var response
+    try {
+        response = fetch(event.request)
+    } catch (error) {
+        response = new Response(
+            "<h1>No network connection</h1>Retrying...<script>setTimeout(() => { window.location.reload(1); }, 5000);</script>",
+            {
+                headers: {
+                    'Content-type': 'text/html'
+                }
+            })
+    }
+    event.respondWith(response);
 });

cmd/fastgallery/main.go

@@ -1355,6 +1355,7 @@ func main() {
 
 		// Copy PWA web manifest and fill-in relevant details
 		createPWAManifest(gallery, source, args.DryRun, config)
+		// TODO move asset creation with HTML and do version comparison
 
 		// Handle ctrl-C or other signals
 		setupSignalHandler()