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.'