core: Send hardcoded User-Agent header in requests

This is required for the server to automatically mark
sent messages as read. See `sent_by_human` in
`zulip/zerver/models.py`.

Fixes: zulip#440

lib/api/core.dart

@@ -69,6 +69,10 @@ class ApiConnection {
     }
   }
 
+  void addUserAgent(http.BaseRequest request) {
+    request.headers['User-Agent'] = 'ZulipMobile/?.?.? (Android 14)';
+  }
+
   final http.Client _client;
 
   bool _isOpen = true;
@@ -80,6 +84,7 @@ class ApiConnection {
     assert(debugLog("${request.method} ${request.url}"));
 
     addAuth(request);
+    addUserAgent(request);
 
     final http.StreamedResponse response;
     try {