fix initial state of connection popup

nicegui/templates/index.html

@@ -37,7 +37,7 @@
     {{ body_html | safe }}
 
     <div id="app"></div>
-    <div id="popup">
+    <div id="popup" aria-hidden="true">
       <span>Connection lost.</span>
       <span>Trying to reconnect...</span>
     </div>