Only estimate remaining after first audit, not from purchase date

Before any audit, bulk items show their full original weight with no
decay applied. The linear decay model only kicks in after the first
audit provides an actual data point. This prevents freshly added,
unopened items from showing misleading consumption estimates.

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

web/src/components/ProductDetail.tsx

@@ -294,7 +294,7 @@ export function ProductDetail({
                   {cfg?.unit}). Re-audit to update.
                 </div>
               )}
-              {item.containerWeight != null && (
+              {item.containerWeight != null && last && (
                 <div
                   style={{
                     fontSize: 11,