Root cause: handleAddNode/handleEdgeConfirm only updated Zustand store,
never creating records in the DB. canvasApi.save() only updated positions
of existing nodes, so nothing survived a page refresh.
Fix: save now sends the full canvas state (all nodes + edges + data) and
the backend does a full sync — upsert incoming, delete anything removed.
This means Save is the single source of truth: no need to call individual
create/update/delete APIs for every drag or edit.
Pouzor
9eba62c5b5