feat: add side-by-side item comparison for duplicate Part Numbers

New Component: ItemComparisonModal.tsx
- Shows existing vs new item side-by-side
- Highlights fields that are different (in yellow)
- Options to Update item or Skip (local-only save)
- Shows existing item ID and comparison details

Backend Changes:
- Updated error message to say 'Part Number' not 'barcode'
- 409 response includes existing item data for comparison
- Clear, user-friendly conflict messaging

Frontend Changes:
- New state for comparison modal (newItem, existingItem, existingId)
- handleOnboardingComplete() shows modal on 409 conflict
- handleComparisonUpdate() calls updateItem() API
- handleComparisonSkip() saves locally without syncing
- Better error handling distinguishes 409 from other failures

Workflow:
1. User imports item with Part Number that already exists
2. System shows comparison modal
3. User can:
   - Update (merges new data into existing)
   - Skip (saves locally, doesn't sync to cloud)

frontend/app/page.tsx

@@ -7,6 +7,7 @@ import { fetchAndCacheItems, syncOfflineOperations } from '@/lib/sync';
 import Scanner from '@/components/Scanner';
 import AIOnboarding from '@/components/AIOnboarding';
 import PageShell from '@/components/PageShell';
+import ItemComparisonModal from '@/components/ItemComparisonModal';
 import { toast } from 'react-hot-toast';
 import {
   Package,
@@ -90,6 +91,8 @@ export default function Home() {
   const [currentUser, setCurrentUser] = useState<any | null>(null);
   const [categories, setCategories] = useState<any[]>([]);
   const [fieldScanning, setFieldScanning] = useState<{ active: boolean, field: string } | null>(null);
+  const [comparisonModal, setComparisonModal] = useState<{ show: boolean, newItem: any, existingItem: any, existingId: number | null }>({ show: false, newItem: null, existingItem: null, existingId: null });
+  const [comparisonLoading, setComparisonLoading] = useState(false);
 
   useEffect(() => {
     if (!localStorage.getItem('inventory_token')) {
@@ -169,31 +172,68 @@ export default function Home() {
         try {
           await inventoryApi.createItem(currentUser.id, itemData);
           toast.success("Item saved to cloud catalog!");
+          setShowOnboarding(false);
+          await loadInventory();
         } catch (err: any) {
           const status = err.response?.status;
-          const detail = err.response?.data?.detail || err.message;
-          console.error("Cloud sync failed:", detail);
+          const responseData = err.response?.data;
 
-          if (status === 409) {
-            toast.error(`Item already exists. ${detail}`);
+          if (status === 409 && responseData?.detail) {
+            // Show comparison modal for duplicate part number
+            const detail = responseData.detail;
+            setComparisonModal({
+              show: true,
+              newItem: itemData,
+              existingItem: detail.existing_item,
+              existingId: detail.existing_id
+            });
+            return; // Don't close onboarding, user will decide
           } else {
+            console.error("Cloud sync failed:", err.message);
             toast.error("Item saved locally. Cloud sync failed.");
+            setShowOnboarding(false);
+            await loadInventory();
           }
         }
       } else if (!isOnline) {
         toast.success("Item saved locally. Will sync when online.");
+        setShowOnboarding(false);
+        await loadInventory();
       } else {
         toast("Please log in to save to cloud.", { icon: '⚠️' });
       }
-
-      setShowOnboarding(false);
-      await loadInventory();
     } catch (error) {
       console.error(error);
       toast.error("Failed to save item");
     }
   };
 
+  const handleComparisonUpdate = async () => {
+    if (!comparisonModal.existingId) return;
+    setComparisonLoading(true);
+    try {
+      // Update existing item
+      await inventoryApi.updateItem(comparisonModal.existingId, comparisonModal.newItem);
+      toast.success("Item updated successfully!");
+      setComparisonModal({ show: false, newItem: null, existingItem: null, existingId: null });
+      setShowOnboarding(false);
+      await loadInventory();
+    } catch (err: any) {
+      console.error("Update failed:", err.message);
+      toast.error("Failed to update item");
+    } finally {
+      setComparisonLoading(false);
+    }
+  };
+
+  const handleComparisonSkip = () => {
+    // Item was already added to local DB, just close modal and onboarding
+    setComparisonModal({ show: false, newItem: null, existingItem: null, existingId: null });
+    setShowOnboarding(false);
+    toast("Local copy saved. Not synced to cloud.", { icon: '💾' });
+    loadInventory();
+  };
+
   const handleSync = useCallback(async () => {
     if (!isOnline || !currentUser) return;
     setSyncing(true);
@@ -616,6 +656,16 @@ export default function Home() {
           />
         )}
 
+        {/* Item Comparison Modal (for duplicate Part Numbers) */}
+        <ItemComparisonModal
+          show={comparisonModal.show}
+          existingItem={comparisonModal.existingItem}
+          newItem={comparisonModal.newItem}
+          onUpdate={handleComparisonUpdate}
+          onSkip={handleComparisonSkip}
+          loading={comparisonLoading}
+        />
+
         {/* Stock Adjustment Overlay */}
         {selectedItem && (
           <div className="fixed inset-0 z-50 flex items-end sm:items-center justify-center p-4 bg-slate-950/80 animate-in fade-in duration-200">