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/access.extensions.ts

@@ -0,0 +1,14 @@
+// Identity hook for build-time access-predicate extensions. Tooling may
+// overwrite this file to widen predicates; the original is restored on
+// cleanup. Mirrors `config/routes.extensions.ts`.
+export type AccessPredicates = {
+  canSeeAdmin: boolean;
+  canSeeOrgAdmin: boolean;
+  canManageCurrentOrg: boolean;
+  canSeeUser: boolean;
+  canDelete: boolean;
+  canLogin: boolean;
+};
+
+export const applyAccessExtensions = <T extends AccessPredicates>(base: T): T =>
+  base;