Use admin-set sale end dates in countdown banner

- Countdown now uses actual sale_end_date from database instead of hardcoded Sunday
- Shows time until earliest sale end date when multiple sales are active
- Falls back to next Sunday if no end dates are set
- Banner now reflects real deadlines set in admin panel

app/page.tsx

@@ -178,6 +178,15 @@ export default function Home() {
         <SaleCountdown 
           hasActiveSale={filaments.some(f => f.sale_active === true)}
           maxSalePercentage={Math.max(...filaments.filter(f => f.sale_active).map(f => f.sale_percentage || 0), 0)}
+          saleEndDate={(() => {
+            const activeSales = filaments.filter(f => f.sale_active && f.sale_end_date);
+            if (activeSales.length === 0) return null;
+            return activeSales.reduce((earliest, current) => {
+              if (!earliest.sale_end_date) return current;
+              if (!current.sale_end_date) return earliest;
+              return new Date(current.sale_end_date) < new Date(earliest.sale_end_date) ? current : earliest;
+            }).sale_end_date;
+          })()}
         />
         
         <FilamentTableV2