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/provider-catalog.tsx

@@ -68,7 +68,8 @@ interface ProviderCatalogProps {
   onSelect?: (provider: string, item: any) => void;
   groupIcons?: Record<string, string>;
   cols?: number;
-  current?: ProviderType | string;
+  // Single value (legacy) or array of selected keys for multi-select.
+  current?: ProviderType | string | string[];
   clickable?: boolean;
   height: string | number;
   showTooltip?: boolean;
@@ -149,7 +150,11 @@ const ProviderCatalog: React.FC<ProviderCatalogProps> = ({
                   <TemplateCard
                     height={height}
                     onClick={() => onSelect?.(action.key as string, action)}
-                    active={current === action.key}
+                    active={
+                      Array.isArray(current)
+                        ? current.includes(action.key)
+                        : current === action.key
+                    }
                     disabled={action.disabled}
                     clickable={clickable}
                     header={renderTitle(action)}