test: Website Item (Item full page)

- Tests for website item. Desk tests and portal tests
- Allow guests on method `get_offer_details`
- Fetch stock details only if `show_stock_availability` is enabled
- Validation for duplicate web items on item merge
- Separate method for `validate_properties_before_merge`
- Common error title and exception `DataValidationError` for merge validations on Item

erpnext/templates/generators/item/item_configure.js

@@ -201,7 +201,7 @@ class ItemConfigure {
 				<span class="mr-2">
 					${frappe.utils.icon('assets', 'md')}
 				</span>
-				${__("Add to Cart")}s
+				${__("Add to Cart")}
 			</button>
 		` : '';