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/hooks/use-cluster.ts

@@ -0,0 +1,130 @@
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
+import { apiClient } from '@/lib/api-client';
+import { nodeKeys } from './use-nodes';
+import { vmKeys } from './use-instances';
+
+// ==================== Types ====================
+
+export interface ClusterServerEntry {
+  id: number;
+  name: string;
+  hostname: string;
+  ip_address: string;
+  port: number;
+  is_online: boolean;
+  cluster_name: string | null;
+}
+
+export interface ClusterTopology {
+  clusters: Record<string, ClusterServerEntry[]>;
+  standalone: ClusterServerEntry[];
+}
+
+export interface PreJoinGuest {
+  vmid: number;
+  name: string;
+  status: string;
+  type: 'qemu' | 'lxc';
+  maxdisk: number;
+  mem: number;
+}
+
+export interface PreJoinStorage {
+  storage: string;
+  type: string;
+  avail: number;
+  total: number;
+}
+
+export interface PreJoinCheck {
+  server_id: number;
+  server_name: string;
+  node_name: string;
+  guests_count: number;
+  guests: PreJoinGuest[];
+  backup_storages: PreJoinStorage[];
+  ready_for_join: boolean;
+  warning: string | null;
+}
+
+export interface PrepareJoinResult {
+  backed_up: number[];
+  deleted: number[];
+  errors: { vmid: number; phase: string; error: string }[];
+  ready_for_join: boolean;
+  message: string;
+}
+
+export interface ClusterActionResult {
+  success: boolean;
+  cluster_name?: string;
+  upid?: string;
+  message: string;
+}
+
+// ==================== Queries ====================
+
+export function useClusterTopology() {
+  return useQuery({
+    queryKey: nodeKeys.topology,
+    queryFn: () => apiClient.get<ClusterTopology>('/proxmox/api/cluster/topology'),
+    refetchInterval: 30000,
+  });
+}
+
+// ==================== Mutations ====================
+
+function useInvalidateCluster() {
+  const qc = useQueryClient();
+  return () => {
+    qc.invalidateQueries({ queryKey: nodeKeys.topology });
+    qc.invalidateQueries({ queryKey: nodeKeys.servers });
+    qc.invalidateQueries({ queryKey: vmKeys.all });
+  };
+}
+
+export function useCreateCluster() {
+  const invalidate = useInvalidateCluster();
+  return useMutation({
+    mutationFn: (body: { server_id: number; cluster_name: string; link0_ip?: string; rootpw?: string }) =>
+      apiClient.post<ClusterActionResult>('/proxmox/api/cluster/create', body),
+    onSuccess: () => invalidate(),
+  });
+}
+
+/** On-demand pre-join inspection of a standalone node (lists guests + backup storages). */
+export function usePreJoinCheck() {
+  return useMutation({
+    mutationFn: (serverId: number) =>
+      apiClient.get<PreJoinCheck>(`/proxmox/api/cluster/${serverId}/pre-join-check`),
+  });
+}
+
+/** Back up and delete all guests on a node so it can join a cluster. Long-running. */
+export function usePrepareJoin() {
+  return useMutation({
+    mutationFn: (body: { node_server_id: number; backup_storage: string; compress?: string }) =>
+      apiClient.post<PrepareJoinResult>('/proxmox/api/cluster/prepare-join', body),
+  });
+}
+
+export function useJoinCluster() {
+  const invalidate = useInvalidateCluster();
+  return useMutation({
+    mutationFn: (body: { node_server_id: number; cluster_server_id: number; rootpw: string; link0?: string }) =>
+      apiClient.post<ClusterActionResult>('/proxmox/api/cluster/join', body),
+    onSuccess: () => invalidate(),
+  });
+}
+
+export function useEjectNode() {
+  const invalidate = useInvalidateCluster();
+  return useMutation({
+    mutationFn: ({ serverId, ...body }: { serverId: number; node_name: string; rootpw?: string; clear_panel_entry?: boolean }) =>
+      apiClient.post<{ success: boolean; ejected_node: string; message: string }>(
+        `/proxmox/api/cluster/${serverId}/eject-node`,
+        body,
+      ),
+    onSuccess: () => invalidate(),
+  });
+}