Phase 13: multi-host networks & images (0.14.0)

Networks and Images are now per-host, rendered as a section for the local host
plus one per registered agent (like the Stacks page).

- agent_app.py: new /agent/networks (list/inspect/containers/connect/disconnect/
  create/delete/prune) and /agent/images (list/updates/check), reusing
  network_service and a new image_service; DockerError mapped to HTTP status
  (forbidden -> 400 so the proxy doesn't treat it as a token failure).
- routers/agents.py: proxy routes at /api/agents/{id}/networks/* and
  /api/agents/{id}/images/*, audit-logging mutations.
- services/image_service.py: extracted the image-listing logic so the central
  router and the agent share it.
- Frontend: networksApi/imagesApi take an optional agentId; Networks/Images
  pages render NetworksSection/ImagesSection per host with a shared HostHeader.
  Remote "Prune unused" networks resolves the address-pool-exhaustion deploy
  error from the UI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/pages/Dashboard.tsx

@@ -17,7 +17,7 @@ export function Dashboard() {
   const stacks = useQuery({ queryKey: ["stacks"], queryFn: stacksApi.list, refetchInterval: 5000 });
   const info = useQuery({ queryKey: ["system"], queryFn: systemApi.info, refetchInterval: 5000 });
   const audit = useQuery({ queryKey: ["audit"], queryFn: () => systemApi.audit(10), refetchInterval: 10000 });
-  const updates = useQuery({ queryKey: ["image-updates"], queryFn: imagesApi.updates, refetchInterval: 60000 });
+  const updates = useQuery({ queryKey: ["image-updates"], queryFn: () => imagesApi.updates(), refetchInterval: 60000 });
 
   const updateCount = Object.values(updates.data ?? {}).filter((u) => u.update_available).length;