feat: editable node groups (add/remove members, description)

Make Node Groups mutable instead of fixed-at-creation:
- removeFromGroup / addToGroup store actions (inverse pair, history-aware)
- right panel: per-member remove button + editable group description
  (reuses data.notes, no backend/serializer change)
- drag a node over a group → confirm modal to add it (getIntersectingNodes)

Tests: +21 (store, panel, canvas detection, modal).

ha-relevant: yes

frontend/src/components/modals/__tests__/ConfirmAddToGroupModal.test.tsx

@@ -0,0 +1,38 @@
+import { describe, it, expect, vi } from 'vitest'
+import { render, screen, fireEvent } from '@testing-library/react'
+import { ConfirmAddToGroupModal } from '../ConfirmAddToGroupModal'
+
+describe('ConfirmAddToGroupModal', () => {
+  it('renders nothing when closed', () => {
+    render(
+      <ConfirmAddToGroupModal open={false} nodeLabel="Router" groupLabel="DMZ" onConfirm={vi.fn()} onCancel={vi.fn()} />,
+    )
+    expect(screen.queryByText('Add to group')).toBeNull()
+  })
+
+  it('shows node and group labels when open', () => {
+    render(
+      <ConfirmAddToGroupModal open nodeLabel="Router" groupLabel="DMZ" onConfirm={vi.fn()} onCancel={vi.fn()} />,
+    )
+    expect(screen.getByText('Router')).toBeDefined()
+    expect(screen.getByText('DMZ')).toBeDefined()
+  })
+
+  it('calls onConfirm when the confirm button is clicked', () => {
+    const onConfirm = vi.fn()
+    render(
+      <ConfirmAddToGroupModal open nodeLabel="Router" groupLabel="DMZ" onConfirm={onConfirm} onCancel={vi.fn()} />,
+    )
+    fireEvent.click(screen.getByRole('button', { name: /add to group/i }))
+    expect(onConfirm).toHaveBeenCalledOnce()
+  })
+
+  it('calls onCancel when the cancel button is clicked', () => {
+    const onCancel = vi.fn()
+    render(
+      <ConfirmAddToGroupModal open nodeLabel="Router" groupLabel="DMZ" onConfirm={vi.fn()} onCancel={onCancel} />,
+    )
+    fireEvent.click(screen.getByRole('button', { name: /cancel/i }))
+    expect(onCancel).toHaveBeenCalledOnce()
+  })
+})