fix: positions totals row aligned with DataTable columns, restore $ prefix

frontend/src/components/DataTable.tsx

@@ -17,6 +17,7 @@ interface Props<T> {
   onRowClick?: (row: T) => void;
   filterFn?: (row: T, query: string) => boolean;
   emptyMessage?: string;
+  footerRow?: ReactNode;
 }
 
 export function DataTable<T extends object>({
@@ -27,6 +28,7 @@ export function DataTable<T extends object>({
   onRowClick,
   filterFn,
   emptyMessage = 'No data',
+  footerRow,
 }: Props<T>) {
   const [sortKey, setSortKey] = useState<string | null>(null);
   const [sortDir, setSortDir] = useState<'asc' | 'desc'>('asc');
@@ -122,6 +124,11 @@ export function DataTable<T extends object>({
               ))
             )}
           </tbody>
+          {footerRow && (
+            <tfoot className="border-t border-surface-700 bg-surface-900">
+              {footerRow}
+            </tfoot>
+          )}
         </table>
       </div>