added OAuth2 redirect fallback message to notify the user to go back to the app

ui/src/components/records/PageOAuth2Redirect.svelte

@@ -6,4 +6,7 @@
     });
 </script>
 
-<div class="block txt-hint txt-center">Loading...</div>
+<div class="content txt-hint txt-center p-base">
+    <h3 class="m-b-sm">Auth completed.</h3>
+    <h5>You can go back to the app if this window is not automatically closed.</h5>
+</div>