UX: revisit model search categories being filters

Currently, the Sketchfab search uses the categories as search scopes/filters instead of top level search queries as is done in Hubs. This is useful if there are scenarios where you want to do a free form search within a specific category, but the downside is that users can often not realize the search is scoped. For example, if you go into the search box now, it defaults to featured, and if you type into the search box, you will get no results (and have to clear the "Featured" filter in order to get results.)

Consider an alternative design (like in Hubs) where the category/featured selections are just queries, not filters. In practice, it seems unlikely people will want to search within categories on Sketchfab and Poly since the search indexes are relatively small and the recall seems subjectively good without needing to filter.

Start by reviewing the current Sketchfab search box and category behavior, then compare it with the Hubs search pattern described in the issue. Done should mean categories such as Featured behave as top-level queries rather than persistent filters, while free-form searches return results without first clearing a category.

Written by the indexing model from the issue text.