fix(canvas): make collapse reachable + persist for every node type

Three connected bugs in PR #158's collapse feature:

1. Toggle wired to the wrong component
   The chevron was on GroupRectNode and computed children via React Flow
   parentId. But in this codebase parentId is set by createGroup() on
   type=group nodes, not on groupRect zones — zones are decorative
   rectangles. Result: childrenCount was always 0 on every zone and the
   button never rendered, so the feature was unreachable from the UI.

   Fix:
   - Add the same chevron toggle to GroupNode (the actual container).
     parentId children are already known there, so the existing BFS in
     computeCollapseInfo hides them when data.collapsed flips.
   - For GroupRectNode, switch childrenCount to spatial containment so
     drawn zones also work: hit-test other nodes' bbox centres against
     the zone bbox.

2. Visibility filter ignored spatial zones
   Extend computeCollapseInfo with a second pass that hides every node
   whose centre lies inside a collapsed groupRect, plus the parentId
   subtrees of those nodes (so a Proxmox host inside a collapsed zone
   takes its VMs/LXCs with it). Edge rewiring routes vanished endpoints
   to the same visible zone via a unified hiddenBy map populated by both
   passes.

3. Save dropped data.collapsed for every type except groupRect
   The DevTools payload was the smoking gun: for a type=group node the
   serializer wrote custom_colors: {show_border: true} with no collapsed
   key, so the backend stored a stale false on every save. Only the
   groupRect branch of serializeNode/deserializeApiNode stashed and
   hoisted the flag. Move the stash + hoist to the general branch too
   (backend's custom_colors is dict[str, Any] so no schema change).

Tests: 11 new cases for spatial containment + GroupNode toggle UI, and
4 round-trip cases for collapse on non-groupRect types.

frontend/src/components/canvas/nodes/GroupRectNode.tsx

@@ -2,6 +2,7 @@ import { useState } from 'react'
 import { Handle, Position, NodeResizer, type NodeProps, type Node } from '@xyflow/react'
 import { ChevronDown } from 'lucide-react'
 import { useCanvasStore } from '@/stores/canvasStore'
+import { getZoneSpatialChildren } from '@/utils/collapseFilter'
 import type { NodeData, TextPosition } from '@/types'
 
 const FONT_FAMILIES: Record<string, string> = {
@@ -54,8 +55,12 @@ export function GroupRectNode({ id, data, selected }: NodeProps<Node<NodeData>>)
   const textPos = (rc.text_position ?? 'top-left') as TextPosition
   const posStyle = POSITION_STYLES[textPos]
 
-  // Count children for collapse badge
-  const childrenCount = (nodes ?? []).filter((n) => n.parentId === id).length
+  // Count children for collapse badge — groupRect zones don't parent their
+  // contents via React Flow parentId, so we hit-test by spatial containment.
+  const selfNode = (nodes ?? []).find((n) => n.id === id)
+  const childrenCount = selfNode
+    ? getZoneSpatialChildren(selfNode, nodes ?? []).length
+    : 0
 
   const outsideJustify = textPos.includes('right') ? 'flex-end'
     : (textPos.includes('center') || textPos === 'center') ? 'center'