The explore page sidebar was showing tags from the current user's private
memos because the default ListMemos query applies a server-side OR filter
(creator_id == X || visibility in [...]), mixing private content in.

Fix by using a visibility-scoped ListMemos request in the explore context
so private memos are always excluded via the AND'd server auth filter.

Also consolidate two always-firing useMemos calls into one context-aware
query, unify activity stats computation with countBy across all branches,
and extract a toDateString helper to remove duplicated formatting logic.