feat: add collapsible/expandable zones for canvas decluttering

- Add collapsed state to NodeData.custom_colors (type=groupRect only)
- Implement toggleNodeCollapsed action in canvasStore (Zustand)
- Extend GroupRectNode UI with smooth chevron toggle button
  - Rotating chevron icon (↓ → when collapsed)
  - Shows '+N' badge when zone is hidden
  - Reduces zone opacity to 0.6 when collapsed
  - All transitions target 60 FPS (ease-out 200ms)
- Filter child nodes/edges in CanvasContainer based on parent collapse state
  - Breadth-first traversal handles multi-level nesting
  - Connecting edges to hidden nodes are automatically hidden
- Add comprehensive test coverage
  - Store: toggleNodeCollapsed state mutation, unsaved flag
  - Component: chevron rendering, click handlers, opacity transitions
- Persist collapsed state via YAML serialization (part of custom_colors)

Benefits:
- Declutter large Zigbee meshes, multi-building networks
- Preserve layout structure without deleting nodes
- Smooth 60 FPS transitions for UX polish

CONTRIBUTING.md compliance:
- Strict TypeScript types, no 'any'
- Zustand store pattern, no prop drilling
- Tests for store logic and component behavior
- Frontend linting requirements met

Co-authored-by: CyberClaw <noreply@openclaw.ai>

frontend/src/types/index.ts

@@ -96,6 +96,8 @@ export interface NodeData extends Record<string, unknown> {
     show_border?: boolean
     width?: number
     height?: number
+    // Collapsible zone state (type === 'groupRect')
+    collapsed?: boolean
   }
   custom_icon?: string
   /** Number of bottom connection points, 1..48. Default 1 (centered). */