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/app.css

@@ -119,6 +119,8 @@ a.button { display: inline-flex; align-items: center; justify-content: center; t
 :is(button, a.button):hover { background: var(--surface-alt); color: var(--light); border-color: var(--outline); }
 :is(button, a.button):active { transform: translateY(1px); }
 :is(button, a.button):focus-visible { background: var(--light); color: var(--ink); border-color: var(--light); outline: none; }
+/* A folder, crumb or space that rows dragged from the list would land in. */
+.carry-over { outline: 1px dashed var(--light); outline-offset: -1px; border-radius: var(--radius-s); }
 button:disabled { color: var(--muted); cursor: default; }
 
 input, textarea {