The sidebar page tree previously kept its expanded/collapsed state in an
in-memory jotai atom, so it reset on every reload. Persist it in
localStorage instead, scoped by `${workspaceId}:${userId}` so multiple
accounts sharing a browser origin don't leak tree state into each other.
- open-tree-nodes-atom: replace the in-memory atom with a facade over an
atomFamily of atomWithStorage, scoped via currentUserAtom; keep the same
[OpenMap, functional-updater setter] public API so space-tree needs no
change.
- Use an explicit synchronous createJSONStorage + getOnInit: true so the
saved state is read at atom init — no collapse-then-expand flicker on
reload and no write against an un-hydrated empty map.
- README / README.ru: document persistent page-tree state in the
"What's different from Docmost" table.
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