Product imports now find collections by title

You can now reference collections by their display title when importing products – no need to convert titles to handles first.

🐛 Fixes

• Custom Collections lookup – importing products with collection titles (e.g., “Baby Toys”) now correctly finds existing collections by converting to handle format (“baby-toys”) instead of creating duplicates.
• New collections – when a collection doesn’t exist, the app creates it with the proper slugified handle while preserving the original title you provided.