Disable start/call button if offline

ts/hooks/useIsOnline.ts

@@ -0,0 +1,26 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { useEffect, useState } from 'react';
+
+export function useIsOnline(): boolean {
+  const [isOnline, setIsOnline] = useState(navigator.onLine);
+
+  useEffect(() => {
+    const update = () => {
+      setIsOnline(navigator.onLine);
+    };
+
+    update();
+
+    window.addEventListener('offline', update);
+    window.addEventListener('online', update);
+
+    return () => {
+      window.removeEventListener('offline', update);
+      window.removeEventListener('online', update);
+    };
+  }, []);
+
+  return isOnline;
+}