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.
