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/GroupNode.tsx

@@ -1,13 +1,14 @@
 import { useState } from 'react'
 import { type NodeProps, type Node, NodeResizer, Handle, Position } from '@xyflow/react'
-import { Layers, Pencil, Check, X } from 'lucide-react'
+import { Layers, Pencil, Check, X, ChevronDown } from 'lucide-react'
 import { useCanvasStore } from '@/stores/canvasStore'
 import { useThemeStore } from '@/stores/themeStore'
 import { THEMES } from '@/utils/themes'
 import { STATUS_COLORS, type NodeData } from '@/types'
 
 export function GroupNode({ id, data, selected }: NodeProps<Node<NodeData>>) {
-  const { nodes, updateNode, snapshotHistory } = useCanvasStore()
+  const { nodes, updateNode, snapshotHistory, toggleNodeCollapsed } = useCanvasStore()
+  const isCollapsed = data.collapsed ?? false
   const activeTheme = useThemeStore((s) => s.activeTheme)
   const theme = THEMES[activeTheme]
   const showBorder = data.custom_colors?.show_border !== false
@@ -138,6 +139,28 @@ export function GroupNode({ id, data, selected }: NodeProps<Node<NodeData>>) {
             </button>
           )}
 
+          {/* Collapse / expand toggle */}
+          {children.length > 0 && (
+            <button
+              className="nodrag"
+              onClick={(e) => { e.stopPropagation(); toggleNodeCollapsed(id) }}
+              title={isCollapsed ? `Show ${children.length} hidden items` : `Hide ${children.length} items`}
+              style={{
+                color: '#00d4ff',
+                background: 'none',
+                border: 'none',
+                cursor: 'pointer',
+                padding: 1,
+                display: 'flex',
+                alignItems: 'center',
+                transition: 'transform 0.2s ease-out',
+                transform: isCollapsed ? 'rotate(-90deg)' : 'rotate(0deg)',
+              }}
+            >
+              <ChevronDown size={11} />
+            </button>
+          )}
+
           {/* Status summary */}
           {children.length > 0 && (
             <div style={{ display: 'flex', alignItems: 'center', gap: 6, fontSize: 10, flexShrink: 0, marginLeft: 4 }}>