I imported a 3.5mm Stereo AudioJack Eagle library, but what category do I put it in? How can I create a “Connectors → Audio” category?
I just added a new category “Barrel Connectors (Audio, Power Jacks, …)”. You should update the “LibrePCB Base” library to get it.
If you ever need to create a category: Open your local library in the library editor and create a new “Component Category” there.