feat(18.1-01): task 4 flat reorder in the admin SPA

- ReorderView reads flat from the reorder data; ReorderBranch takes a flat
  prop and shows only move up and move down
- ArrowRight and ArrowLeft do nothing, an inside drop lands after the row
  (tree.ts flatZone, flatMove) and flatten sends parent_id null
- vitest flat cases; docs describe the flat page; rebuilt dist

docs/backend/lists-and-filters.md

@@ -241,6 +241,8 @@ The reorder is flat when the file names no `parentColumn` and the controller's r
 
 The list schema's `reorder` and the data of both reorder routes carry `flat: true`; a tree reorder omits the key, so its payloads are unchanged. Every node of a flat reorder has `parent_id: null`. A save writes only the sort column, without running the model's save hooks. A node whose `parent_id` is not null is a 422 `validation_failed` on `nodes` with `reorder.flat_parent`; the duplicate-id 422 and the 409 `reorder_stale` set rule are the same as on a tree.
 
+In the admin, a flat reorder page is a plain ordered list: every row has the drag handle and the move up and move down buttons, but no outdent or indent buttons; ArrowUp and ArrowDown on a handle move the row, ArrowRight and ArrowLeft do nothing, and a row dragged onto the middle of another lands after it. Saving works as on a tree, with `parent_id: null` on every node.
+
 ## Remembered list state
 
 The admin keeps a list's search, sort, filters, page and page size in the URL, so a filtered list can be bookmarked or shared. It also remembers the last of these per list in the browser's local storage. When an administrator opens a list without any of these parameters in the URL (from the navigation, a breadcrumb or a form's back, save or delete), the admin restores the remembered query. A URL that carries list parameters wins and replaces the remembered query.