remark-gfm passes checked as undefined for unchecked task items, so the
Base UI Checkbox mounted uncontrolled and permanently ignored the
checked=true arriving after "Check all tasks" rewrote the memo content.
Coerce the prop to a boolean so the checkbox is always controlled.
Also drop the `${updateTime}` suffixes from memo React keys and the
comment editor cache key: a protobuf-es Timestamp stringifies to
"[object Object]", so the suffix was a constant — the keys were
effectively name-only already, and now say so honestly.
Fixesusememos/memos#6143
