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/stores/canvasStore.ts

@@ -379,16 +379,7 @@ export const useCanvasStore = create<CanvasState>((set) => ({
     set((state) => ({
       nodes: state.nodes.map((n) =>
         n.id === id
-          ? {
-              ...n,
-              data: {
-                ...n.data,
-                custom_colors: {
-                  ...n.data.custom_colors,
-                  collapsed: !n.data.custom_colors?.collapsed,
-                },
-              },
-            }
+          ? { ...n, data: { ...n.data, collapsed: !n.data.collapsed } }
           : n
       ),
       hasUnsavedChanges: true,