feat(nodes): hide hub-only views when active node is remote (#1007)

* feat(nodes): hide hub-only views when active node is remote

Fleet, Schedules, Audit, Logs, and Auto-Update operate on hub-owned state
(node registry, fleet schedules, centralized audit, fleet-wide log
aggregation, fleet-wide update preview). When the active node is remote,
proxying those surfaces would show that remote's own disconnected state
instead of the hub's. Hide them from the nav strip and force-redirect to
Home if one was open during the node switch.

Backend hubOnlyGuard middleware sits between nodeContextMiddleware and the
remote proxy and rejects /api/scheduled-tasks, /api/audit-log, and
/api/notification-routes with 403 + HUB_ONLY_ENDPOINT when nodeId resolves
to a remote, closing the script-bypass path the UI gating cannot reach.

Settings sub-sections were already gated via the hiddenOnRemote registry;
this extends the same model to top-level views.

* docs(nodes): note hub-only visibility on Fleet, Schedules, Audit, Logs, Auto-Update

Each of the five hub-only feature pages now points readers to the
canonical "What top-level views show when a remote node is active"
section in multi-node.mdx, so users landing directly on a feature page
understand why the nav item disappears when they switch to a remote node.

docs/features/audit-log.mdx

@@ -7,6 +7,10 @@ description: Track all mutating actions across your Sencho instance with a searc
   The Audit Log requires a Sencho **Admiral** license. Skipper and Community Edition do not include this feature.
 </Note>
 
+<Note>
+  Audit is hub-only and is hidden from the nav strip when a remote node is the active selection. See [Multi-Node Management](/features/multi-node#what-top-level-views-show-when-a-remote-node-is-active).
+</Note>
+
 Sencho Admiral records every mutating action (deploy, stop, delete, settings changes, user management) with full attribution. The audit log answers the question every team eventually asks: **"Who changed what, and when?"**
 
 ## What gets logged