Improve digital products search and filtering

Merchants with large catalogs need better search and filtering in the digital products list: - Filter to show only products without files attached (critical for setup and auditing — merchants with 500+ products need to see at a glance what's missing) - Filter or search by vendor, SKU, or sequential number to find products faster - Better search results when linking products — currently too many similar-named results with no vendor or distinguishing info shown All of these make it much easier to manage and audit large digital product catalogs.