refactor(canvas): promote collapsed to first-class NodeData field + edge rewire

Three follow-ups to PR #158 review:

1. Promote collapsed to NodeData.collapsed
   The flag was previously stashed inside NodeData.custom_colors, which is
   a colors/style object — semantically wrong. Move it to a first-class
   boolean on NodeData. Persistence keeps the existing API shape: serialize
   writes it into the custom_colors blob (alongside width/height/z_order,
   matching how groupRect already stashes layout metadata), and deserialize
   hoists it back. Legacy saves from the original PR shape load correctly.

2. Re-route cross-zone edges to the collapsed ancestor
   Previously any edge touching a hidden node was dropped, so a Zigbee
   coordinator outside a collapsed mesh lost all visible links to it.
   rewireEdgesForCollapse now walks each endpoint up the parentId chain to
   its nearest visible ancestor, surfaces a single stub edge on the
   collapsed zone, de-dupes parallel rewires (a 20-device mesh becomes one
   stub, not twenty), and drops edges that would self-loop on a zone or
   reference an orphan.

3. Revert package-lock.json churn
   The 63-line diff from the original PR was npm-version drift (libc
   arrays stripped from optional deps), unrelated to the feature.

Tests:
- canvasStore.collapse: updated to assert on data.collapsed.
- collapseFilter: 8 cases for visibility + 7 for edge rewire, covering
  cross-boundary, nested collapse, sibling self-loop, mesh dedup, and
  orphan endpoints.
- canvasSerializer.collapse: round-trip + legacy-shape compat.

frontend/src/types/index.ts

@@ -96,9 +96,13 @@ export interface NodeData extends Record<string, unknown> {
     show_border?: boolean
     width?: number
     height?: number
-    // Collapsible zone state (type === 'groupRect')
-    collapsed?: boolean
   }
+  /**
+   * Collapsible zone state (type === 'groupRect'). When true, the zone hides
+   * its descendants on the canvas. Persisted via `custom_colors.collapsed`
+   * round-trip for back-compat with older saves.
+   */
+  collapsed?: boolean
   custom_icon?: string
   /** Number of bottom connection points, 1..48. Default 1 (centered). */
   bottom_handles?: number