resetItems/removeUnseen/the UnseenItemsEvent handler each did `new Map(prevValue)`
(a shallow copy) then spliced/pushed the per-category array returned by
`.get(category)` — the SAME array reference still held by the previous Map. That
mutates state outside React's data flow (breaks under StrictMode double-invoke and
any updater replay). resetItems additionally did `splice(existing.indexOf(id), 1)`
with no guard, so an id not present (indexOf === -1) spliced off the wrong LAST
element. Replace each in-place splice/push with a cloned array set back on the new
Map (filter for removals, spread+push for the merge).
simoleo89