Passed to CategorySwitcher by FeedTabBar, ProfileDumpsTab,
ProfilePlaylistsTab, Playlists and Search, which keep the category in
their URL.
The set it offers: dump categories for dumps, playlist ones for playlists.
optional
categorySlug: string
The category shown, or none for every dump or playlist.
onSelect: (slug: string | undefined) => void
Called with the slug of the category picked, or undefined for "All".