feat(web/mazenet): show target host in topology list + war map

Renders the swarm host (or "master") that a topology is deployed to,
both as a meta line on each topology list card and in the war-map
header. Operators can now distinguish master-local from agent-targeted
topologies at a glance — previously the only signal was the abstract
"mode: agent" label, with no hint of which agent.

Adds useSwarmHosts() hook for the uuid → host lookup. Falls back to a
short uuid prefix when the hosts list is unavailable so the UI never
hard-fails on a missing /swarm/hosts response.

TopologySummary gains target_host_uuid in the frontend type so the
field actually narrows when checked.

decnet_web/src/hooks/useSwarmHosts.ts

@@ -0,0 +1,42 @@
+import { useCallback, useEffect, useState } from 'react';
+import api from '../utils/api';
+
+export interface SwarmHost {
+  uuid: string;
+  name: string;
+  address: string;
+  agent_port: number;
+  status: string;
+  last_heartbeat: string | null;
+}
+
+/**
+ * Lookup of enrolled swarm hosts. One-shot fetch on mount, with a manual
+ * refresh callback. Used to resolve `target_host_uuid` → display name in
+ * places where we don't already have a host name in hand (topology list,
+ * war-map header).
+ *
+ * Failure is treated as "no agents enrolled" — callers fall back to the
+ * uuid prefix or a generic label rather than blocking on this lookup.
+ */
+export function useSwarmHosts(): {
+  hosts: SwarmHost[];
+  byUuid: Map<string, SwarmHost>;
+  refresh: () => Promise<void>;
+} {
+  const [hosts, setHosts] = useState<SwarmHost[]>([]);
+
+  const refresh = useCallback(async () => {
+    try {
+      const { data } = await api.get<SwarmHost[]>('/swarm/hosts');
+      setHosts(data ?? []);
+    } catch {
+      setHosts([]);
+    }
+  }, []);
+
+  useEffect(() => { refresh(); }, [refresh]);
+
+  const byUuid = new Map(hosts.map((h) => [h.uuid, h]));
+  return { hosts, byUuid, refresh };
+}