feat(picker): New folder button for Create New's custom folder picker

Turns on the picker's New folder button for "Choose the folder to create the
case in", the folder picker behind Create New's custom folder option.

src/web/public/session-ui.js

@@ -4051,6 +4051,7 @@ Object.assign(CodemanApp.prototype, {
       title: 'Choose the folder to create the case in',
       initialPath: input.value.trim(),
       directoriesOnly: true,
+      allowNewFolder: true,
       onSelect: (path) => {
         input.value = path;
         this.updateNewCasePathPreview();