fix(friends): not able to sort by 'friends for' column (@fehmer) (#7057)

Author: fehmer

frontend/src/html/pages/friends.html

@@ -59,7 +59,7 @@
         <thead>
           <tr>
             <td data-sort-property="name">name</td>
-            <td data-sort-property="addedAt">friends for</td>
+            <td data-sort-property="lastModified">friends for</td>
             <td data-sort-property="xp">level</td>
             <td
               data-sort-property="completedTests"