Autofill next asset ID and pre-select bin from existing inventory

Asset tag field now defaults to the highest existing asset ID + 1.
Bin selector pre-selects the bin where other instances of the same
product are stored, falling back to the first bin.

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

web/src/components/modals/AddInventoryFlow.tsx

@@ -358,10 +358,18 @@ function InstanceDetailsStep({
     return last.price;
   })();
 
-  const [assetId, setAssetId] = useState("");
+  const nextAssetId = useMemo(() => {
+    const max = data.inventoryItems.reduce(
+      (m, i) => Math.max(m, parseInt(i.assetId, 10) || 0),
+      0,
+    );
+    return max > 0 ? String(max + 1).padStart(6, "0") : "";
+  }, [data.inventoryItems]);
+
+  const [assetId, setAssetId] = useState(nextAssetId);
   const [form, setForm] = useState({
     shopId: last?.shopId ?? data.shops[0]?.id ?? NEW_SHOP,
-    binId: data.bins[0]?.id ?? NEW_BIN,
+    binId: last?.binId ?? data.bins[0]?.id ?? NEW_BIN,
     weight: last?.weight ?? (isDiscrete ? 0 : 3.5),
     unitWeight: last?.unitWeight ?? (isDiscrete ? 0.7 : 0),
     price: initialPrice,