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/backends/index.tsx

@@ -166,7 +166,15 @@ const BackendList = () => {
     }
     // ================ Delete ================
     if (item.action === 'delete') {
-      if (item.data.backend_source === BackendSourceValueMap.COMMUNITY) {
+      // Platform community rows aren't actually deletable — the
+      // "delete" action there is a soft-disable on the admin-curated
+      // catalog. An org-scoped row of any source (including an org's
+      // override of a community backend) IS owner-mutable data and
+      // gets a real DELETE.
+      const isPlatformCommunity =
+        item.data.backend_source === BackendSourceValueMap.COMMUNITY &&
+        item.data.organization_id == null;
+      if (isPlatformCommunity) {
         modalRef.current?.show({
           content: 'backends.title',
           operation: 'common.delete.single.confirm',