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/check-environment.tsx

@@ -11,6 +11,7 @@ const CheckEnvironment: React.FC<AddWorkerStepProps> = ({ disabled }) => {
   const { stepList, summary, provider } = useAddWorkerContext();
   const intl = useIntl();
   const currentGPU = summary.get('currentGPU');
+  const currentGPUs: string[] = summary.get('selectedGPUs') || [];
   const workerCommand = summary.get('workerCommand') || {
     label: '',
     link: '',
@@ -51,7 +52,11 @@ const CheckEnvironment: React.FC<AddWorkerStepProps> = ({ disabled }) => {
       <Typography.Paragraph style={{ marginBottom: 8 }}>
         {intl.formatMessage({ id: 'cluster.create.checkEnv.tips' })}
       </Typography.Paragraph>
-      <CheckEnvCommand provider={provider} currentGPU={currentGPU} />
+      <CheckEnvCommand
+        provider={provider}
+        currentGPU={currentGPU}
+        currentGPUs={currentGPUs}
+      />
     </StepCollapse>
   );
 };