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/ConfirmAddToGroupModal.tsx

@@ -0,0 +1,47 @@
+import { Layers } from 'lucide-react'
+import {
+  Dialog,
+  DialogContent,
+  DialogHeader,
+  DialogTitle,
+  DialogDescription,
+  DialogFooter,
+} from '@/components/ui/dialog'
+import { Button } from '@/components/ui/button'
+
+interface ConfirmAddToGroupModalProps {
+  open: boolean
+  nodeLabel: string
+  groupLabel: string
+  onConfirm: () => void
+  onCancel: () => void
+}
+
+export function ConfirmAddToGroupModal({ open, nodeLabel, groupLabel, onConfirm, onCancel }: ConfirmAddToGroupModalProps) {
+  return (
+    <Dialog open={open} onOpenChange={(o) => { if (!o) onCancel() }}>
+      <DialogContent className="max-w-sm">
+        <DialogHeader>
+          <DialogTitle className="flex items-center gap-2">
+            <Layers size={16} className="text-[#00d4ff]" />
+            Add to group
+          </DialogTitle>
+          <DialogDescription>
+            Add <span className="font-medium text-foreground">{nodeLabel}</span> to the group{' '}
+            <span className="font-medium text-foreground">{groupLabel}</span>?
+          </DialogDescription>
+        </DialogHeader>
+        <DialogFooter>
+          <Button variant="ghost" size="sm" onClick={onCancel}>Cancel</Button>
+          <Button
+            size="sm"
+            className="bg-[#00d4ff] text-[#0d1117] hover:bg-[#00d4ff]/90"
+            onClick={onConfirm}
+          >
+            Add to group
+          </Button>
+        </DialogFooter>
+      </DialogContent>
+    </Dialog>
+  )
+}