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/support-gpus.tsx

@@ -76,14 +76,19 @@ const ProviderImage = ({ src, height }: { src: string; height?: number }) => {
 
 interface SupportedHardwareProps {
   onSelect?: (provider: string, item: any) => void;
-  current?: string;
+  // Single (legacy) or array of selected GPU driver keys (multi-select).
+  current?: string | string[];
   clickable?: boolean;
+  // Set of GPU driver keys that are valid to pick. When provided, items
+  // outside this set render as disabled. Undefined means "no restriction".
+  availableKeys?: Set<string>;
 }
 
 const SupportedHardware: React.FC<SupportedHardwareProps> = ({
   onSelect,
   clickable,
-  current
+  current,
+  availableKeys
 }) => {
   const intl = useIntl();
   const { userSettings } = useUserSettings();
@@ -199,13 +204,20 @@ const SupportedHardware: React.FC<SupportedHardwareProps> = ({
     }
   ];
 
+  const platformsWithDisabled = availableKeys
+    ? supportedHardPlatforms.map((p) => ({
+        ...p,
+        disabled: !availableKeys.has(p.value)
+      }))
+    : supportedHardPlatforms;
+
   return (
     <Box className={userSettings?.theme === 'realDark' ? 'dark-theme' : ''}>
       <ProviderCatalog
         onSelect={onSelect}
         height={60}
         current={current}
-        dataList={supportedHardPlatforms}
+        dataList={platformsWithDisabled}
         clickable={clickable}
         showTooltip={true}
         cols={5}