feat: plugin extension slots for access, request interceptors, Users actions, and form fields

Four purely-additive seams that let build-time tooling extend host
behaviour without forking files:

* `src/access.extensions.ts` — identity `applyAccessExtensions`
  (mirrors `config/routes.extensions.ts`). `src/access.ts` runs the
  resolved predicate object through it. Adds two new predicates,
  `canSeeOrgAdmin` and `canManageCurrentOrg`, alongside the existing
  `canSeeAdmin`. Resources, Models children, Cluster Management, and
  Resources/Backends are retagged from `canSeeAdmin` to
  `canSeeOrgAdmin`. Users and Dashboard stay strict.
* `src/request.extensions.ts` — identity-empty
  `extraRequestInterceptors`. `src/request-config.tsx` spreads it
  into the existing `requestInterceptors` list so extensions can
  inject context-aware headers without forking the request config.
* Users page action column — renders
  `getGPUStackPlugin()?.components?.UserRowActions` next to the
  existing DropdownButtons inside a Space when a plugin component
  is registered. If absent, the cell renders exactly as before.
* `src/components/plugin-extra-fields.tsx` — generic component-slot
  helper. Renders `pluginManager.components.<name>` if registered,
  forwarding a `context` payload. Used by create/edit forms to let
  plugins inject extra `Form.Item` fields. Mounted in the relevant
  create forms — API Keys, Cloud Credentials, Clusters, Model
  Routes, Model Providers, and Inference Backends — under the slot
  name `CreateOrgScopeField`. Resources whose org is implicit from a
  parent (Models / Workers / Benchmarks / Worker Pools / Model Files
  inherit from the chosen Cluster) deliberately don't mount the slot.

src/pages/model-routes/forms/basic.tsx

@@ -1,16 +1,20 @@
+import PluginExtraFields from '@/components/plugin-extra-fields';
 import CategorySelect from '@/pages/_components/category-select';
 import DocLink from '@/pages/_components/doc-link';
 import { categoryOptions } from '@/pages/llmodels/config';
 import { CheckboxField, Input as CInput, useAppUtils } from '@gpustack/core-ui';
 import { useIntl } from '@umijs/max';
 import { Form } from 'antd';
+import { useContext } from 'react';
 import { genericReferLink } from '../config';
+import FormContext from '../config/form-context';
 import { FormData } from '../config/types';
 
 const Basic = () => {
   const intl = useIntl();
   const form = Form.useFormInstance<FormData>();
   const { getRuleMessage } = useAppUtils();
+  const { action } = useContext(FormContext);
   return (
     <>
       <Form.Item
@@ -28,6 +32,7 @@ const Basic = () => {
           label={intl.formatMessage({ id: 'common.table.name' })}
         />
       </Form.Item>
+      <PluginExtraFields name="CreateOrgScopeField" context={{ action }} />
       <Form.Item
         name="categories"
         normalize={(value) => (value ? [value] : [])}