kueue: add MultiKueueConfig list and detail views

[Thesmoothengineer]

What this PR does

Adds initial Headlamp UI support for Kueue's MultiKueueConfig resource, following the same
pattern established by ClusterQueue, LocalQueue, ResourceFlavor, Workload, and
ProvisioningRequestConfig:

• src/resources/multiKueueConfig.ts — MultiKueueConfig resource class with a spec.clusters
  getter.
• src/resources/multiKueueConfigFormatters.ts — exported, documented, pure formatter functions
  for rendering the cluster list and cluster count.
• src/resources/multiKueueConfig.test.ts — unit tests for the formatters.
• src/components/multikueueconfigs/List.tsx and Detail.tsx — list and detail views.
• Registers a new MultiKueueConfigs sidebar entry and routes in index.tsx /
  kueueRoutes.ts.

Testing

• npm run test — all 38 tests pass (5 new).
• npm run build — builds cleanly.
• Verified locally in a running Headlamp instance against a kind cluster with a real
  MultiKueueConfig resource — both the list and detail views render the cluster names and
  cluster count correctly.

Why

MultiKueue is called out as a potential focus area for this term. This PR adds basic
visibility into MultiKueueConfig resources as a foundation that later work (e.g. manager/
worker cluster relationships, connectivity, workload routing) can build on.