feat: extend cluster k8s_options with multi-vendor manifest support

- New k8s_pod_spec form sections (image credentials, node selector,
  gpu vendor overrides) under k8s_options, replacing the legacy flat
  k8s_volume_mounts list. UI keys aligned to the backend wire shape
  (snake_case k8s_options + camelCase inside).
- System default container registry is pre-filled into the first image
  credential when creating a new cluster; empty username/password are
  coerced to null on submit to match the Optional[str] backend schema.
- Register cluster flow supports multi-runtime selection gated by the
  cluster's gpuVendorOverrides: non-override vendors stay single-select
  with an inline hint; multi-add only opens once an override vendor is
  picked, and non-override cards become disabled in that state.
- Manifest URL emits multiple ?runtime= params; check-env step combines
  per-vendor commands; downstream steps are disabled when no vendor is
  selected.
- Pre-validate gpuVendorOverrides at save time (non-empty selector, no
  duplicates across vendors, no key clash with base nodeSelector) so
  the user sees the error before hitting the manifest endpoint.
- Misc: dark-mode background of the k8s_pod_spec / volume mount titles
  no longer clashes with the drawer; cluster Steps no longer leaks the
  internal showModules/showForms props to the DOM.

src/pages/cluster-management/components/add-worker/k8s-run-command.tsx

@@ -12,6 +12,7 @@ const K8sRunCommand: React.FC<AddWorkerStepProps> = ({ disabled }) => {
 
   const stepIndex = stepList.indexOf(StepNamesMap.RunCommand) + 1;
   const currentGPU = summary.get('currentGPU') || '';
+  const currentGPUs: string[] = summary.get('selectedGPUs') || [];
 
   return (
     <StepCollapse
@@ -36,6 +37,7 @@ const K8sRunCommand: React.FC<AddWorkerStepProps> = ({ disabled }) => {
       <RegisterClusterInner
         registrationInfo={registrationInfo}
         currentGPU={currentGPU}
+        currentGPUs={currentGPUs}
       />
     </StepCollapse>
   );