feat: show each account's last sign-in in Users

A right-aligned column at the end of every Users & roles row says how long ago the
account signed in, with the exact moment as the hint and as a line in the account
panel; a dash where nothing is recorded, and the demo account says it is not recorded.
formatDateTime formats that moment in the UI language rather than the browser's.

frontend/src/components/AdminUsers.tsx

@@ -24,6 +24,7 @@ import {
   type PendingAccessRow,
 } from "../lib/api";
 import { notify } from "../lib/notifications";
+import { formatDateTime, relativeTime } from "../lib/format";
 import { invalidateUserAccess } from "../lib/adminUsers";
 import { ADMIN_USERS_TAB_KEY } from "../lib/adminUsersTab";
 import { Button } from "./ui/Button";
@@ -310,6 +311,23 @@ function UsersRoles({ me }: { me: Me }) {
                     icon={<Trash2 className="w-4 h-4" />}
                   />
                 </Tooltip>
+                {/* How long ago this account last signed in. LAST in the row on purpose: the badges and
+                    the role button before it vary in width row to row, so only the row's own right
+                    edge lines the values up into one column. The exact moment is the hint here and a
+                    line in the account panel; below `sm` the row has no room, and the panel still says it. */}
+                <Tooltip
+                  hint={
+                    u.last_login_at
+                      ? t("users.roles.lastLogin", { time: formatDateTime(u.last_login_at) })
+                      : u.is_demo
+                        ? t("users.roles.lastLoginDemo")
+                        : t("users.roles.lastLoginNone")
+                  }
+                >
+                  <span className="hidden sm:inline-block w-20 shrink-0 text-right text-2xs text-muted tabular-nums">
+                    {u.last_login_at ? relativeTime(u.last_login_at) : "—"}
+                  </span>
+                </Tooltip>
               </div>
             );
           })}