try websocket and only fall back on long polling

default was to first start long polling and then upgrade to websocket
which is hurting performance and produces errors in edge cases

nicegui/templates/index.html

@@ -155,8 +155,9 @@
           const query = { client_id: "{{ client_id }}" };
           const url = window.location.protocol === 'https:' ? 'wss://' : 'ws://' + window.location.host;
           const extraHeaders = {{ socket_io_js_extra_headers | safe }};
+          const transports = ['websocket', 'polling'];
           window.path_prefix = "{{ prefix | safe }}";
-          window.socket = io(url, { path: "{{ prefix | safe }}/_nicegui_ws/socket.io", query, extraHeaders });
+          window.socket = io(url, { path: "{{ prefix | safe }}/_nicegui_ws/socket.io", query, extraHeaders, transports });
           window.socket.on("connect", () => {
             window.socket.emit("handshake", (ok) => {
               if (!ok) window.location.reload();