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.
