feat: drop node onto container node to nest it

Dropping a top-level node over any container_mode node (proxmox,
docker_host, ...) now pops a confirm modal that nests it as a child
(sets parentId), mirroring the existing drop-onto-group flow.

- store: addToContainer(containerId, childId)
- CanvasContainer: detect container_mode intersection on drag stop
  (group still wins if both intersect)
- generalize ConfirmAddToGroupModal with a container variant

ha-relevant: yes

frontend/src/stores/__tests__/canvasStore.test.ts

@@ -578,6 +578,79 @@ describe('canvasStore', () => {
     expect(useCanvasStore.getState().hasUnsavedChanges).toBe(true)
   })
 
+  // ── addToContainer ──────────────────────────────────────────────────────────
+
+  it('addToContainer nests a top-level node under a container_mode node', () => {
+    const container = { ...makeNode('px1', { type: 'proxmox', container_mode: true, label: 'PX' }), position: { x: 76, y: 52 }, width: 448, height: 252 }
+    const child = { ...makeNode('n1'), position: { x: 300, y: 200 } }
+    useCanvasStore.setState({ nodes: [container, child] })
+
+    useCanvasStore.getState().addToContainer('px1', 'n1')
+
+    const moved = useCanvasStore.getState().nodes.find((n) => n.id === 'n1')
+    expect(moved?.parentId).toBe('px1')
+    expect(moved?.extent).toBe('parent')
+    expect(moved?.data.parent_id).toBe('px1')
+    // 300-76=224, 200-52=148
+    expect(moved?.position).toEqual({ x: 224, y: 148 })
+  })
+
+  it('addToContainer works for any container_mode type (docker_host)', () => {
+    const host = { ...makeNode('dh1', { type: 'docker_host', container_mode: true }), position: { x: 0, y: 0 } }
+    const child = { ...makeNode('n1'), position: { x: 50, y: 50 } }
+    useCanvasStore.setState({ nodes: [host, child] })
+
+    useCanvasStore.getState().addToContainer('dh1', 'n1')
+
+    expect(useCanvasStore.getState().nodes.find((n) => n.id === 'n1')?.parentId).toBe('dh1')
+  })
+
+  it('addToContainer places the container before the child in the array', () => {
+    const child = { ...makeNode('n1'), position: { x: 300, y: 200 } }
+    const container = { ...makeNode('px1', { type: 'proxmox', container_mode: true }), position: { x: 0, y: 0 } }
+    useCanvasStore.setState({ nodes: [child, container] })
+
+    useCanvasStore.getState().addToContainer('px1', 'n1')
+
+    const { nodes } = useCanvasStore.getState()
+    expect(nodes.findIndex((n) => n.id === 'px1')).toBeLessThan(nodes.findIndex((n) => n.id === 'n1'))
+  })
+
+  it('addToContainer is a no-op when target is not in container_mode', () => {
+    const notContainer = { ...makeNode('px1', { type: 'proxmox', container_mode: false }), position: { x: 0, y: 0 } }
+    const child = { ...makeNode('n1'), position: { x: 50, y: 50 } }
+    useCanvasStore.setState({ nodes: [notContainer, child] })
+    useCanvasStore.getState().markSaved()
+
+    useCanvasStore.getState().addToContainer('px1', 'n1')
+
+    expect(useCanvasStore.getState().nodes.find((n) => n.id === 'n1')?.parentId).toBeUndefined()
+    expect(useCanvasStore.getState().hasUnsavedChanges).toBe(false)
+  })
+
+  it('addToContainer is a no-op when child already belongs to the container', () => {
+    const container = { ...makeNode('px1', { type: 'proxmox', container_mode: true }), position: { x: 0, y: 0 } }
+    const child = { ...makeNode('n1'), position: { x: 50, y: 50 }, parentId: 'px1', extent: 'parent' as const }
+    useCanvasStore.setState({ nodes: [container, child] })
+    useCanvasStore.getState().markSaved()
+
+    useCanvasStore.getState().addToContainer('px1', 'n1')
+
+    expect(useCanvasStore.getState().hasUnsavedChanges).toBe(false)
+  })
+
+  it('addToContainer snapshots history and marks unsaved', () => {
+    const container = { ...makeNode('px1', { type: 'proxmox', container_mode: true }), position: { x: 0, y: 0 } }
+    const child = { ...makeNode('n1'), position: { x: 50, y: 50 } }
+    useCanvasStore.setState({ nodes: [container, child] })
+    useCanvasStore.getState().markSaved()
+
+    useCanvasStore.getState().addToContainer('px1', 'n1')
+
+    expect(useCanvasStore.getState().past).toHaveLength(1)
+    expect(useCanvasStore.getState().hasUnsavedChanges).toBe(true)
+  })
+
   // ── removeFromGroup ─────────────────────────────────────────────────────────
 
   it('removeFromGroup releases the child to absolute coords and keeps the group', () => {