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/__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()
+  })
 })