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/panels/__tests__/GroupPanels.test.tsx

@@ -42,6 +42,7 @@ const mockStore = {
   snapshotHistory: vi.fn(),
   createGroup: vi.fn(),
   ungroup: vi.fn(),
+  removeFromGroup: vi.fn(),
 }
 
 function setupStore(overrides = {}) {
@@ -230,4 +231,45 @@ describe('GroupDetailPanel', () => {
     fireEvent.click(screen.getByText('Child Node Alpha'))
     expect(setSelectedNode).toHaveBeenCalledWith('c1')
   })
+
+  it('removes a child from the group via the remove button', () => {
+    const removeFromGroup = vi.fn()
+    const snapshotHistory = vi.fn()
+    const group = makeGroupNode()
+    const child = makeNode('c1', { parentId: 'g1', data: { label: 'Router', type: 'router', status: 'online', services: [] } })
+    setupStore({ nodes: [group, child], selectedNodeId: 'g1', selectedNodeIds: ['g1'], removeFromGroup, snapshotHistory })
+    renderPanel()
+    fireEvent.click(screen.getByRole('button', { name: /remove router from group/i }))
+    expect(removeFromGroup).toHaveBeenCalledWith('g1', 'c1')
+    expect(snapshotHistory).toHaveBeenCalled()
+  })
+
+  it('renders the existing group description from notes', () => {
+    const group = makeGroupNode()
+    group.data = { ...group.data, notes: 'Critical DMZ hosts' } as typeof group.data
+    setupStore({ nodes: [group], selectedNodeId: 'g1', selectedNodeIds: ['g1'] })
+    renderPanel()
+    expect((screen.getByLabelText('Description') as HTMLTextAreaElement).value).toBe('Critical DMZ hosts')
+  })
+
+  it('commits the description on blur', () => {
+    const updateNode = vi.fn()
+    const snapshotHistory = vi.fn()
+    const group = makeGroupNode()
+    setupStore({ nodes: [group], selectedNodeId: 'g1', selectedNodeIds: ['g1'], updateNode, snapshotHistory })
+    renderPanel()
+    const textarea = screen.getByLabelText('Description')
+    fireEvent.change(textarea, { target: { value: 'New notes' } })
+    fireEvent.blur(textarea)
+    expect(updateNode).toHaveBeenCalledWith('g1', { notes: 'New notes' })
+  })
+
+  it('does not commit the description on blur when unchanged', () => {
+    const updateNode = vi.fn()
+    const group = makeGroupNode()
+    setupStore({ nodes: [group], selectedNodeId: 'g1', selectedNodeIds: ['g1'], updateNode })
+    renderPanel()
+    fireEvent.blur(screen.getByLabelText('Description'))
+    expect(updateNode).not.toHaveBeenCalled()
+  })
 })