fix: the editor shows an empty page, not the last file, while a file loads; text in UTF-16 and with BOM; + menu groups open and shut smoothly; the assistant's errors in plain words

internal/server/web/templates/layout.html

@@ -1,5 +1,6 @@
 {{define "head"}}<meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
+<link rel="stylesheet" href="{{asset "codemirror/codemirror.css"}}">
 <link rel="stylesheet" href="{{asset "app.css"}}">
 <script src="{{asset "pin.js"}}"></script>
 <script src="{{asset "scroll.js"}}" defer></script>{{end}}