fix(tree): address realtime-tree-server review findings

- make addTreeNode receivers idempotent (invalidateOnCreatePage guard +
  buildTree dedup) so the author's self-echo no longer duplicates the node
- broadcast realtime tree updates for bulk copy/duplicate and import via a
  root refetch: PAGE_CREATED now carries spaceId and the WS listener falls
  back to refetchRootTreeNodeEvent when no per-node snapshot is present
- remove the now-dead client-relay inbound path (isTreeEvent/handleTreeEvent)
  that remained a stale-restriction-cache attack surface
- honest string|null cast for a root move's parent id
- add tests: buildTree dedup; onPageCreated per-node vs refetch branching

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/page/tree/utils/utils.ts

@@ -29,7 +29,14 @@ export function buildTree(pages: IPage[]): SpaceTreeNode[] {
     };
   });
 
+  // Defense-in-depth: a duplicate id in `pages` would push two references to the
+  // same node, producing a duplicate React key that crashes the sidebar render.
+  // Track ids we've already pushed and skip repeats so a stray duplicate from a
+  // realtime cache write can never break the tree.
+  const seen = new Set<string>();
   pages.forEach((page) => {
+    if (seen.has(page.id)) return;
+    seen.add(page.id);
     tree.push(pageMap[page.id]);
   });