Add OS to User-Agent

ts/util/getUserAgent.ts

@@ -1,6 +1,20 @@
 // Copyright 2020 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
+import { getOwn } from './getOwn';
+
+const PLATFORM_STRINGS: { [platform: string]: string } = {
+  win32: 'Windows',
+  darwin: 'macOS',
+  linux: 'Linux',
+};
+
 export function getUserAgent(appVersion: string): string {
-  return `Signal-Desktop/${appVersion}`;
+  // `process.platform` could be missing if someone figures out how to compile Signal on
+  //   an unsupported OS and forgets to update this file. We'd rather send nothing than
+  //   crash.
+  const platformString = getOwn(PLATFORM_STRINGS, process.platform);
+  const platformStringWithSpace = platformString ? ` ${platformString}` : '';
+
+  return `Signal-Desktop/${appVersion}${platformStringWithSpace}`;
 }