Customer FAQ Explorer shows categories without visible articles

[sgrallert]

Description:
Currently, the FAQ Explorer in the customer portal displays all categories, even when:

• The category contains no FAQ articles at all
• All articles in the category are not visible to the specific customer (due to permissions or state)

Expected behavior:
Categories without any visible articles for the customer should be hidden from the FAQ Explorer.

Possible solution:
Add a SysConfig option to enable hiding empty categories in the customer FAQ Explorer.