feat: sync session warning across tabs

backend/workflows/templates/workflows/developer_handbook.html

@@ -846,6 +846,7 @@ make backup-verify BACKUP_DIR=backups/backup_YYYYmmdd_HHMMSS</code></pre>
         <li><code>SESSION_IDLE_TIMEOUT_SECONDS</code> controls the idle session window.</li>
         <li><code>SENSITIVE_ACTION_REAUTH_SECONDS</code> controls when sensitive POST actions require fresh authentication.</li>
         <li><code>/session/keepalive/</code> refreshes both session timestamps when the user chooses <code>Angemeldet bleiben</code>.</li>
+        <li>Open tabs now sync the confirmed session timestamp through browser storage and <code>BroadcastChannel</code>, so extending the session in one tab updates the warning state in the others.</li>
       </ul>
       <p>This warning is meant to protect work in progress without silently relaxing the security middleware.</p>
     </div>