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/websocket/use-tree-socket.ts

@@ -1,6 +1,6 @@
 import { useEffect } from "react";
 import { socketAtom } from "@/features/websocket/atoms/socket-atom.ts";
-import { useAtom } from "jotai";
+import { useAtom, useSetAtom } from "jotai";
 import { treeDataAtom } from "@/features/page/tree/atoms/tree-data-atom.ts";
 import { WebSocketEvent } from "@/features/websocket/types";
 import { SpaceTreeNode } from "@/features/page/tree/types.ts";
@@ -16,7 +16,10 @@ import localEmitter from "@/lib/local-emitter.ts";
 
 export const useTreeSocket = () => {
   const [socket] = useAtom(socketAtom);
-  const [, setTreeData] = useAtom(treeDataAtom);
+  // Setter-only: this hook writes the tree from socket events but never reads it
+  // reactively, so useSetAtom avoids re-rendering UserProvider (its host) on
+  // every tree event.
+  const setTreeData = useSetAtom(treeDataAtom);
   const queryClient = useQueryClient();
 
   useEffect(() => {
@@ -37,7 +40,11 @@ export const useTreeSocket = () => {
   }, []);
 
   useEffect(() => {
-    socket?.on("message", (event: WebSocketEvent) => {
+    if (!socket) return;
+    // Named handler + off() cleanup (mirrors use-notification-socket). Without
+    // cleanup, every socket recreation / effect re-run stacked another listener,
+    // so a single broadcast fired duplicated tree walks after each reconnect.
+    const handleMessage = (event: WebSocketEvent) => {
       switch (event.operation) {
         case "updateOne":
           if (event.entity[0] === "pages") {
@@ -64,6 +71,11 @@ export const useTreeSocket = () => {
           });
           break;
       }
-    });
-  }, [socket]);
+    };
+
+    socket.on("message", handleMessage);
+    return () => {
+      socket.off("message", handleMessage);
+    };
+  }, [socket, queryClient, setTreeData]);
 };