feat(client): persist page-tree open/closed state per workspace+user

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.

README.md

@@ -38,6 +38,7 @@ The goal of the fork is a **100% open, AGPL-only build with no Enterprise-Editio
 | **AI agent chat** | Built-in AI agent chat over your wiki, written from scratch as a community feature — no enterprise license. The agent reads and edits pages on your behalf (scoped to your permissions), with full-text + vector (RAG) search and optional web access via external MCP servers. |
 | **Rebranding** | App logo / name changed from *Docmost* to *Gitmost*. |
 | **Compact page tree** | Default page-tree indentation reduced from 16px to 8px per nesting level. |
+| **Persistent page-tree state** | The sidebar page tree remembers which nodes you expanded/collapsed across reloads — saved in the browser (localStorage), scoped per workspace + user so accounts sharing a browser don't clash. Upstream Docmost forgets the tree on every reload. |
 | **CI / images** | Release CI publishes container images to GHCR (`ghcr.io/vvzvlad/gitmost`) using the built-in `GITHUB_TOKEN` instead of Docker Hub. |
 
 ### Embedded MCP server