Number unnamed parts in explore mode

I created a lot of parts and didn't name them, so the next parts dropdown was full of identical "unnamed part" options. Adding a number to the end of "unnamed part" would help to differentiate them.

Start by locating the explore-mode next-parts dropdown and the code that produces the “unnamed part” label. Confirm how unnamed parts are ordered, then make the labels distinguishable with numbers and verify that the dropdown shows the updated names.

Written by the indexing model from the issue text.