feat: add expand/collapse functionality for graph nodes

[ysansan98]

Demo

2025-08-09.00.19.49.mp4

Features

• Support single node expand/collapse (+/- button)
• Support one-click expand/collapse for all nodes

Implementation Summary

• Extended Node interface: added expanded and hasChildren properties
• Added reactive variables: childToParentMap, isFirstCalculateGraph, collapsedNodes, and isUpdating
• Optimized calculateGraph to build parent-child relationships, skip collapsed nodes, and compute hasChildren
• Added toggleNode for handling node expand/collapse events
• Added adjustScrollPositionAfterToggle to preserve scroll position
• Added expandAll / collapseAll batch operation methods

[antfu]

Thanks for working on it! I think it's a great feature to have. However, I think we have to do a bit more improvements:

1. We should have some visual indicator to show if a node is collapsed (or expandable)
2. Seems we need to fix some style (border, background of the nodes)?

[ysansan98]

1. Does this look better?

2. Thank you for the reminder — I missed that earlier. It's now been fixed.

[ysansan98]

hi @antfu , when convenient, please review this PR and share any feedback for adjustments