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.svelte

@@ -568,10 +568,14 @@
         else void workspace.guard(() => workspace.openSelected())
         break
       case 'm':
-        // Pick a node up here, walk to a folder, put it down with `p`.
+        // Pick nodes up here, walk to a folder, put them down with `p`.
         event.preventDefault()
         workspace.mark()
         break
+      case 'M':
+        event.preventDefault()
+        workspace.markAll()
+        break
       case 'p':
         event.preventDefault()
         void workspace.guard(() => workspace.drop())
@@ -619,6 +623,7 @@
       case 'Escape':
         event.preventDefault()
         if (workspace.open) void runCommand('q')
+        else workspace.unmarkAll()
         break
       case 'x':
         event.preventDefault()