🍒category: assert names aren't duplicated

this is an implicit requirement of our sorting logic that can break when categories are non-uniquely translated

specifically it makes no sense to have ambiguous category names on the same "level", the user isn't able to tell them apart (in particular also in a a11y context -- e.g. screenreaders). so it makes no sense at all

guard against this with a simplistic test that loads all categories in all known languages

CCBUG: 481303

(cherry picked from commit 40995bd1)

Co-authored-by: Harald Sitter sitter@kde.org