Revert rota comparison to real oncosts; remove stale NI footnote

Now that the sync uses /api/v2/rosters/on/{date} (2026-07-25) instead
of /api/v2/schedules, real employer-oncost figures are available for
rota data, so gatherRotaVsActualData() no longer needs the base-cost-
only workaround — it respects show_oncosts like the rest of the app
again. Also removes the now-inaccurate Monthly.tsx footnote claiming
rota-based forecasts exclude NI, and the now-obsolete methodology note
that told the model to expect a swing from the base-cost fix (moot —
insight history is being cleared, so there's nothing to compare
against).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/Monthly.tsx

@@ -508,9 +508,6 @@ export default function Monthly() {
               </tbody>
             </table>
             {showOncosts && <p className="footnote">Includes estimated employer on-costs. Final payroll figures are in Sage.</p>}
-            {isCurrentMonth && forecastMethod === 'rota' && (
-              <p className="footnote">Rota-based forecast figures exclude employer National Insurance — Workforce's schedules API doesn't provide it, only the timesheets/actuals API does.</p>
-            )}
           </div>
 
           {pyDepts.length > 0 && (