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/CanvasContainer.tsx

@@ -17,7 +17,7 @@ import '@xyflow/react/dist/style.css'
 import { useCanvasStore } from '@/stores/canvasStore'
 import { useThemeStore } from '@/stores/themeStore'
 import { THEMES } from '@/utils/themes'
-import { getVisibleNodeIds, rewireEdgesForCollapse } from '@/utils/collapseFilter'
+import { computeCollapseInfo, rewireEdgesForCollapse } from '@/utils/collapseFilter'
 import { nodeTypes } from './nodes/nodeTypes'
 import { edgeTypes } from './edges/edgeTypes'
 import { SearchBar } from './SearchBar'
@@ -57,14 +57,14 @@ export function CanvasContainer({ onConnect: onConnectProp, onEdgeDoubleClick, o
   const theme = THEMES[activeTheme]
 
   // Filter nodes and edges based on collapsed state (memoized — O(n)).
-  const visibleNodeIds = useMemo(() => getVisibleNodeIds(nodes), [nodes])
+  const collapseInfo = useMemo(() => computeCollapseInfo(nodes), [nodes])
   const visibleNodes = useMemo(
-    () => nodes.filter((n) => visibleNodeIds.has(n.id)),
-    [nodes, visibleNodeIds],
+    () => nodes.filter((n) => collapseInfo.visibleIds.has(n.id)),
+    [nodes, collapseInfo],
   )
   const visibleEdges = useMemo(
-    () => rewireEdgesForCollapse(edges, nodes, visibleNodeIds),
-    [edges, nodes, visibleNodeIds],
+    () => rewireEdgesForCollapse(edges, nodes, collapseInfo.visibleIds, collapseInfo.hiddenBy),
+    [edges, nodes, collapseInfo],
   )
 
   const onNodeClick = useCallback((e: React.MouseEvent, node: Node<NodeData>) => {

frontend/src/components/canvas/__tests__/GroupNode.test.tsx

@@ -42,6 +42,7 @@ function renderGroupNode(props: Partial<Parameters<typeof GroupNode>[0]> = {}, s
     nodes: storeNodes,
     updateNode: vi.fn(),
     snapshotHistory: vi.fn(),
+    toggleNodeCollapsed: vi.fn(),
   } as unknown as ReturnType<typeof canvasStore.useCanvasStore>)
 
   return render(
@@ -134,4 +135,51 @@ describe('GroupNode', () => {
     renderGroupNode()
     expect(screen.queryByText(/●/)).toBeNull()
   })
+
+  it('renders a collapse toggle when the group has parentId children', () => {
+    const storeNodes = [
+      { id: 'c1', parentId: 'g1', data: { status: 'online' } },
+      { id: 'c2', parentId: 'g1', data: { status: 'online' } },
+    ]
+    renderGroupNode({}, storeNodes)
+    expect(screen.getByTitle('Hide 2 items')).toBeDefined()
+  })
+
+  it('flips the toggle title when collapsed', () => {
+    const storeNodes = [
+      { id: 'c1', parentId: 'g1', data: { status: 'online' } },
+    ]
+    renderGroupNode({ data: makeGroupNode({ collapsed: true }).data }, storeNodes)
+    expect(screen.getByTitle('Show 1 hidden items')).toBeDefined()
+  })
+
+  it('calls toggleNodeCollapsed when the toggle is clicked', () => {
+    const toggleNodeCollapsed = vi.fn()
+    const storeNodes = [{ id: 'c1', parentId: 'g1', data: { status: 'online' } }]
+    vi.mocked(canvasStore.useCanvasStore).mockReturnValue({
+      nodes: storeNodes,
+      updateNode: vi.fn(),
+      snapshotHistory: vi.fn(),
+      toggleNodeCollapsed,
+    } as unknown as ReturnType<typeof canvasStore.useCanvasStore>)
+    render(
+      <GroupNode
+        id="g1"
+        data={makeGroupNode().data}
+        selected={false}
+        dragging={false}
+        zIndex={1}
+        isConnectable={true}
+        positionAbsoluteX={0}
+        positionAbsoluteY={0}
+      />,
+    )
+    fireEvent.click(screen.getByTitle('Hide 1 items'))
+    expect(toggleNodeCollapsed).toHaveBeenCalledWith('g1')
+  })
+
+  it('does not render the toggle when the group has no children', () => {
+    renderGroupNode()
+    expect(screen.queryByTitle(/Hide.*items|Show.*hidden/)).toBeNull()
+  })
 })

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 }}>

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'