feat: users row actions accept plugin entries

The per-row user actions menu was hard-wired to a fixed list and any
plugin-contributed control had to sit as a separate trigger beside
the dropdown. Move the seam: plugins now contribute dropdown items
directly via `users.rowActions`, and we dispatch the click to the
plugin entry by key while the host's `handleSelect` keeps owning
the built-in keys.

Each entry is `{ key, labelId, icon?, danger?, show?(user), onClick(user) }`.
For UI state the onClick might need (drawers, modals), plugins can
mount a single `components.UsersPageGlobal` slot that the page now
renders once outside the table — typical pattern is a jotai atom
the rowAction writes to and the global slot reads from.

Drop the side-mounted `PluginExtraFields name="UserRowActions"` and
its surrounding `<Space>`; the action column is back to a single
DropdownButtons with all entries in one menu.

src/pages/users/index.tsx

@@ -1,3 +1,4 @@
+import PluginExtraFields from '@/components/plugin-extra-fields';
 import { PageAction } from '@/config';
 import { PaginationKey, TABLE_SORT_DIRECTIONS } from '@/config/settings';
 import type { PageActionType } from '@/config/types';
@@ -216,6 +217,11 @@ const Users: React.FC = () => {
         onOk={handleModalOk}
       ></AddModal>
       <DeleteModal ref={modalRef}></DeleteModal>
+      {/* Plugin mount point for global Users-page UI (e.g. a
+          memberships drawer driven from a `users.rowActions`
+          entry's onClick). Renders nothing when no plugin is
+          registered. */}
+      <PluginExtraFields name="UsersPageGlobal" />
     </>
   );
 };