feat(frontend): cluster management page (create / join wizard / eject)

Surface the existing backend cluster API in the UI:
- New /cluster page with topology view (clusters + standalone servers),
  per-node online status, and a 2-node quorum warning.
- Create cluster dialog (node, name with Proxmox naming rules, optional
  corosync link0, optional root password).
- 3-step Join wizard: pick joining + target node → pre-join-check with
  backup-and-clear (prepare-join) for nodes that still hold guests →
  confirm + join with the node root password.
- Eject node dialog (with SSH-fallback root password).
- hooks/use-cluster.ts: typed useClusterTopology, useCreateCluster,
  usePreJoinCheck, usePrepareJoin, useJoinCluster, useEjectNode.
- Nav item + route + RU/EN i18n strings.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/layouts/AppLayout.tsx

@@ -59,6 +59,7 @@ const navGroups: NavGroup[] = [
       { label: 'Dashboard', icon: LayoutDashboard, path: '/dashboard' },
       { label: 'Instances', icon: Monitor, path: '/instances' },
       { label: 'Nodes', icon: Server, path: '/nodes' },
+      { label: 'Cluster', icon: Network, path: '/cluster' },
       { label: 'Templates', icon: HardDrive, path: '/templates' },
       { label: 'Backups', icon: Archive, path: '/backups' },
       { label: 'Tasks', icon: ClipboardList, path: '/tasks' },