perf(client): cut background re-renders + duplicate work (#344)

Outside the editor the UI did background work on every tree event, socket
reconnect, and navigation. Tree infra (virtualization/memo/O(N) utils) was
already good — the cost was in the subscriptions and duplicates around it.
Client-only; behavior 1:1.

- Setter-only atom subscriptions → useSetAtom: space-tree-row, use-tree-mutation,
  use-tree-socket no longer subscribe every visible row to the WHOLE treeDataAtom
  value (a tree event re-rendered all ~20-30 rows, bypassing the DocTreeRow memo).
  space-tree-node-menu / mention-list read the tree imperatively (store.get) in
  their handlers only. breadcrumb.tsx uses a selectAtom slice (ancestor chain +
  field equality) instead of the whole-tree subscription.
- Socket handler cleanup (BUG): use-tree-socket + use-query-subscription now
  socket.off() their named handlers on cleanup (were accumulating listeners on
  every reconnect → duplicated invalidations/tree-walks). Mirrors
  use-notification-socket.
- Field-update tree path: invalidateOnUpdatePage does a pointwise patch of the
  cached embed subtrees instead of a blanket invalidatePageTree() (refetch storm);
  structural events keep the blanket invalidate.
- usePageMetaQuery: a content-less select slice for the 13 peripheral subscribers
  that read only title/permissions/id, so they stop re-rendering every ~3s while
  typing / on every collab page.updated (page.tsx keeps the full query for content).
- page.tsx: skeleton + placeholderData keepPreviousData (no blank flash on nav).
- Removed refetchOnMount:true where socket/mutation invalidation already keeps the
  cache fresh (favorite/space/space-watcher/workspace). KEPT it on the 3 queries
  with NO other freshness path (trash-list, created-by, recent-changes) — the
  global default is refetchOnMount:false, so those overrides are load-bearing.
- Small: resize mousemove/up attached only while dragging; per-row emoji-picker
  keydown gated on `opened`; AiChatWindow queries enabled only when the window is
  open.

Gate: client tsc 0, client vitest page+websocket 200 passed (+editor suites),
build ok.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/page/tree/components/space-tree-node-menu.tsx

@@ -1,4 +1,4 @@
-import { useAtom } from "jotai";
+import { useSetAtom, useStore } from "jotai";
 import { useTranslation } from "react-i18next";
 import { useParams } from "react-router-dom";
 import { ActionIcon, Menu, rem } from "@mantine/core";
@@ -52,7 +52,11 @@ export function NodeMenu({ node, canEdit }: NodeMenuProps) {
   const clipboard = useClipboard({ timeout: 500 });
   const { spaceSlug } = useParams();
   const { handleDelete } = useTreeMutation(node.spaceId);
-  const [data, setData] = useAtom(treeDataAtom);
+  // Setter-only: the tree value is read only imperatively inside the duplicate
+  // handler (via `store` below), never at render, so useSetAtom avoids
+  // re-rendering every row's NodeMenu on any tree event.
+  const setData = useSetAtom(treeDataAtom);
+  const store = useStore();
   const emit = useQueryEmit();
   const [exportOpened, { open: openExportModal, close: closeExportModal }] =
     useDisclosure(false);
@@ -125,8 +129,8 @@ export function NodeMenu({ node, canEdit }: NodeMenuProps) {
     try {
       const duplicatedPage = await duplicatePage({ pageId: node.id });
 
-      // figure out parent + insertion index
-      const siblings = treeModel.siblingsOf(data, node.id);
+      // figure out parent + insertion index (read the live tree imperatively)
+      const siblings = treeModel.siblingsOf(store.get(treeDataAtom), node.id);
       const parentId = siblings?.parentId ?? null;
       const currentIndex = siblings?.index ?? 0;
       const newIndex = currentIndex + 1;