Add PY(TD) sub-label to % of Net Sales summary card

Shows prior year equivalent % (wages/net sales) as a sub-label under the
current period value — labelled PY WTD/MTD for current partial periods,
PY for complete past periods.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/Monthly.tsx

@@ -226,6 +226,9 @@ export default function Monthly() {
         <div className="summary-card">
           <div className="label">% Net Sales</div>
           <div className="value">{pctSales != null ? `${pctSales.toFixed(1)}%` : '—'}</div>
+          {pyWages != null && pySales > 0 && (
+            <div className="sub">{isCurrentMonth ? 'PY MTD' : 'PY'} {((pyWages / pySales) * 100).toFixed(1)}%</div>
+          )}
         </div>
       </div>