Search: menu for categories like for post types

[cellio]

https://meta.codidact.com/posts/284644

When we addressed https://github.com/codidact/qpixel/issues/554, we added a selector for post types (so people wouldn't need to know the exact names and we wouldn't need to handle string parsing), but for users and categories we make you get the ID. For users that clearly makes sense; names aren't unique. For categories, the number is in the URL, but can we offer a menu too? It would need to get the category names and IDs for the current community only, show them to the user, and turn a selection into the number. (I think it'd be good to actually show the numbers along with the names, so people might learn them and type them directly in queries.)

A solution exists, so I'm marking this as low priority, but I'm filing it in case it's easy. (I vaguely recall some discussion of this beyond what's in the linked issue, but if so I can't find it.)