fix: prevent duplicate item creation with barcode conflict detection

Backend:
- Added UNIQUE constraint check on barcode before item creation
- Returns 409 Conflict with user-friendly message if duplicate exists
- Prevents sqlite3.IntegrityError crashes

Frontend:
- Improved error handling for cloud sync failures
- Detects 409 status code (duplicate barcode)
- Shows specific error message to user
- Gracefully falls back to local-only save

Example error message: 'Item already exists. Item with barcode P66093-002 already exists (ID: 42). Update it instead or use a different barcode.'

frontend/app/page.tsx

@@ -170,8 +170,15 @@ export default function Home() {
           await inventoryApi.createItem(currentUser.id, itemData);
           toast.success("Item saved to cloud catalog!");
         } catch (err: any) {
-          console.error("Cloud sync failed:", err.message);
-          toast.error("Item saved locally. Cloud sync failed.");
+          const status = err.response?.status;
+          const detail = err.response?.data?.detail || err.message;
+          console.error("Cloud sync failed:", detail);
+
+          if (status === 409) {
+            toast.error(`Item already exists. ${detail}`);
+          } else {
+            toast.error("Item saved locally. Cloud sync failed.");
+          }
         }
       } else if (!isOnline) {
         toast.success("Item saved locally. Will sync when online.");