WP-12: Kubernetes overview and workload actions

kube-rs gateway (nodes, namespaces, deployments/statefulsets/daemonsets with
images, PVCs; rollout restart, scale, set image via patches) using the
microk8s kubeconfig by default, fake gateway for dev, /api/cluster routes,
Kubernetes page with node cards, workload table with admin actions and PVC
list. Also implements the streaming command runner that WP-11 relied on.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

frontend/src/components/AppShell.vue

@@ -7,6 +7,7 @@ const router = useRouter()
 const nav = [
   { to: '/', label: 'Dashboard' },
   { to: '/updates', label: 'Updates' },
+  { to: '/cluster', label: 'Kubernetes' },
   { to: '/vulnerabilities', label: 'Vulnerabilities' },
   { to: '/backups', label: 'Backups' },
   { to: '/jobs', label: 'Jobs' },