tree: several nodes picked up and moved at once

m now adds the row under the cursor to a set of marks and steps on, so
mmm picks up three; M takes the whole folder, Esc or :unmark puts it all
back. Marks stay while the list walks, so they can be gathered from
several folders. p, :mv and Shift+digit move the whole set.

With a pointer, Ctrl- or Cmd-click marks a row and Shift-click every row
from the cursor to it. Rows drag onto a folder row, a crumb or a space
pill; a marked row carries every marked node with it.

POST /api/nodes/move takes { ids, parent_id } and moves them all or
none: every node is checked before any moves, sealing into a sealed
folder happens before the one transaction that moves them, and two of a
name get slugs of their own. The single move goes the same way.

web/src/lib/api.ts

@@ -359,6 +359,9 @@ export const api = {
 
   moveNode: (id: number, parent_id: number, position?: number) =>
     request<WorkspaceNode>(`/nodes/${id}/move`, { method: 'POST', body: body({ parent_id, position }) }),
+  /** All of them into `parent_id`, or none when one cannot go. */
+  moveNodes: (ids: number[], parent_id: number) =>
+    request<{ moved: number; unchanged: number }>('/nodes/move', { method: 'POST', body: body({ ids, parent_id }) }),
 
   /** Publishes the document, or mints a fresh key for one already published. */
   share: (id: number) =>