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/websocket/use-tree-socket.ts

@@ -115,7 +115,9 @@ export const useTreeSocket = () => {
               | undefined;
             const patch: Partial<SpaceTreeNode> = {
               position: event.payload.position,
-              parentPageId: newParentId as string,
+              // Honest type: a root move has a null parent, so this is
+              // `string | null`, not always `string`.
+              parentPageId: newParentId as string | null,
             };
             if (pageData) {
               // The tree node stores the title as `name`.