feat: lasso selection, multi-select panel, and named groups

- Add lasso/box selection via selectionOnDrag (Space to pan, lasso by default)
- Add lasso/pan toggle button in canvas controls (bottom-left)
- Multi-select panel in right panel when 2+ nodes selected (including zones)
- Create named Group node from selected nodes with bounding box math
- Group node: resizable, inline rename, show/hide border toggle, status summary
- GroupDetailPanel: lists members, online/offline count, ungroup action
- Fix group persistence after save/reload (extend proxmox container map to include group nodes)
- Fix groupRect serialization to preserve parent_id
- Remove background color from group and proxmox container node wrappers
- Add tests for all new store actions, GroupNode, MultiSelectPanel, GroupDetailPanel

frontend/src/utils/canvasSerializer.ts

@@ -63,7 +63,7 @@ export function serializeNode(n: Node<NodeData>): Record<string, unknown> {
       check_target: null,
       services: [],
       notes: null,
-      parent_id: null,
+      parent_id: n.data.parent_id ?? null,
       container_mode: false,
       custom_icon: null,
       pos_x: n.position.x,
@@ -142,6 +142,7 @@ export function deserializeApiNode(
       width: w,
       height: h,
       zIndex: z - 10,
+      ...(n.parent_id ? { parentId: n.parent_id, extent: 'parent' as const } : {}),
     }
   }
   const parentIsContainer = n.parent_id ? (proxmoxContainerMap.get(n.parent_id) ?? false) : false