Don't show the Hide/Show option if it won't work

boards/static/js/main.js

@@ -268,7 +268,8 b' function addContextMenu() {'
                         name: gettext('Hide/show'),
                         callback: function(key, opt) {
                             togglePostHidden(postId);
-                        }
+                        },
+                        visible: !!localStorage
                     },
                     edit: {
                         name: gettext('Edit'),