feat(graph): add collapsible legend with node type visibility toggle

[haodonLiu]

Summary

Enhance Knowledge Graph legend with collapsible panel, node type visibility toggle, and left-side scrollbar.
增强知识图谱图例面板：添加可折叠功能、按类型切换节点可见性、左侧滚动条。

Changes

File	Description
src/components/graph/graph-view.tsx	Legend UI enhancement, HiddenTypeManager, state management
src/index.css	Custom scrollbar styling for legend

Features

Feature	Description
Collapsible Legend	Click ▼/▶ to collapse/expand legend panel
Double-click Hide	Double-click a node type to hide all nodes of that type
Edge Sync	Connected edges auto-hide when nodes are hidden
Show All	"Show all" button restores all hidden types
Left-side Scrollbar	Scrollbar appears on left side for cleaner layout

Interaction

Action	Result
Click ▼/▶ button	Collapse/expand legend
Double-click "Entity"	Hide all Entity nodes + connected edges
Double-click again	Restore visibility
Click "Show all"	Restore all hidden types

Breaking Changes

None

Testing

npm run tauri dev
# Navigate to Knowledge Graph view
# Test collapse/expand, double-click hide, scrollbar  

Screenshots