feat: Add PWA support - manifest, service worker, offline banner

frontend/src/hooks/useOnlineStatus.js

@@ -0,0 +1,18 @@
+import { useState, useEffect } from 'react';
+
+export function useOnlineStatus() {
+  const [isOnline, setIsOnline] = useState(navigator.onLine);
+
+  useEffect(() => {
+    const goOnline = () => setIsOnline(true);
+    const goOffline = () => setIsOnline(false);
+    window.addEventListener('online', goOnline);
+    window.addEventListener('offline', goOffline);
+    return () => {
+      window.removeEventListener('online', goOnline);
+      window.removeEventListener('offline', goOffline);
+    };
+  }, []);
+
+  return isOnline;
+}