[enh] simple theme: mobile layout: allow access to the preferences

when there is no query, reduce the width of the query field,
to display the preference button on the right.

searx/static/themes/simple/src/less/search.less

@@ -195,6 +195,11 @@
     flex: 1;
   }
 
+  #main_results #q:placeholder-shown ~ #send_search {
+    margin-right: 2.6rem;
+    transition: margin-right 0.1s;
+  }
+
   .search_filters {
     display: block;
     margin: 0.8em 0;