Ability to filter / sort on `null_percentage`

In the column explorer, it would be amazing to be able to filter and sort on `null_percentage`. This would allow you to both:

- remove all "noise" and only focus on columns with data
- review all unused columns in your dataset to potentially cleanup

If you REALLY wanted to be fancy, the filter could have a threshold (EG `>95%` null), but honestly just having the sort would be good enough.

Start by locating the column explorer entry point and the code that exposes column metadata, then trace how existing filters and sorting are implemented. Add sorting by `null_percentage` and verify that the explorer orders columns correctly; the optional threshold filter is explicitly not required.

Written by the indexing model from the issue text.