extract the save-bulk handler into a submit closure shared by the
save button click and a keydown listener on both the title and
count inputs. focus the title input as soon as the form opens.
extract the save-child handler into a submit closure shared by
the save button click and a keydown listener on the input. also
focus the input as soon as the form opens so the user can type
and hit enter without touching the mouse.
introduce a module-level expandedNodeIds set that tracks which
nodes the user has manually expanded. renderTree consults the set
when deciding initial visibility (falling back to the depth-based
default for a fresh load), the toggle click handler keeps the set
in sync, and both add-child save handlers add the parent's id
before triggering the re-fetch. on a fresh load the set starts
empty so root-only-open behavior is unchanged and the existing
toggle tests keep passing.
