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/add-worker-step.tsx

@@ -96,6 +96,28 @@ const AddWorkerSteps: React.FC<AddWorkerProps> = (props) => {
     );
   }, [clusterList, stepList, StepNamesMap]);
 
+  // Downstream steps (check env, run command, ...) only make sense after a
+  // GPU vendor has been chosen. If the user toggled off every vendor in
+  // multi-select, gate them shut so the wrong panel can't be opened.
+  const selectedGPUs =
+    (summary.get('selectedGPUs') as string[] | undefined) || [];
+  const currentGPU = (summary.get('currentGPU') as string | undefined) || '';
+  const noVendorSelected = !currentGPU && selectedGPUs.length === 0;
+  const downstreamDisabled = disabled || noVendorSelected;
+
+  React.useEffect(() => {
+    // If the user just deselected everything, collapse any downstream
+    // panel back to the GPU step so they aren't left looking at a stale
+    // disabled-but-open command. Functional updater so we don't have to
+    // depend on `collapseKey` and re-run the effect on every toggle.
+    if (!noVendorSelected) return;
+    setCollapseKey((prev) =>
+      prev.has(StepNamesMap.SelectGPU)
+        ? prev
+        : new Set([StepNamesMap.SelectGPU])
+    );
+  }, [noVendorSelected]);
+
   return (
     <AddWorkerContext.Provider
       value={{
@@ -123,16 +145,20 @@ const AddWorkerSteps: React.FC<AddWorkerProps> = (props) => {
           !stepList.includes(StepNamesMap.SelectCluster)) && (
           <>
             <SelectVendor disabled={disabled}></SelectVendor>
-            <CheckEnvironment disabled={disabled}></CheckEnvironment>
+            <CheckEnvironment disabled={downstreamDisabled}></CheckEnvironment>
 
             {provider === ProviderValueMap.Kubernetes && (
-              <K8sRunCommand disabled={disabled}></K8sRunCommand>
+              <K8sRunCommand disabled={downstreamDisabled}></K8sRunCommand>
             )}
 
             {provider === ProviderValueMap.Docker && (
               <>
-                <SpecifyArguments disabled={disabled}></SpecifyArguments>
-                <DockerRunCommand disabled={disabled}></DockerRunCommand>
+                <SpecifyArguments
+                  disabled={downstreamDisabled}
+                ></SpecifyArguments>
+                <DockerRunCommand
+                  disabled={downstreamDisabled}
+                ></DockerRunCommand>
               </>
             )}
           </>