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/components/panels/__tests__/DetailPanel.test.tsx

@@ -26,10 +26,13 @@ function setupStore(nodeData: Partial<NodeData> = {}) {
   vi.mocked(canvasStore.useCanvasStore).mockReturnValue({
     nodes: [makeNode(nodeData)],
     selectedNodeId: 'n1',
+    selectedNodeIds: [],
     setSelectedNode: vi.fn(),
     deleteNode: vi.fn(),
     updateNode: vi.fn(),
     snapshotHistory: vi.fn(),
+    createGroup: vi.fn(),
+    ungroup: vi.fn(),
   } as unknown as ReturnType<typeof canvasStore.useCanvasStore>)
 }
 
@@ -38,10 +41,13 @@ describe('DetailPanel', () => {
     vi.mocked(canvasStore.useCanvasStore).mockReturnValue({
       nodes: [],
       selectedNodeId: null,
+      selectedNodeIds: [],
       setSelectedNode: vi.fn(),
       deleteNode: vi.fn(),
       updateNode: vi.fn(),
       snapshotHistory: vi.fn(),
+      createGroup: vi.fn(),
+      ungroup: vi.fn(),
     } as unknown as ReturnType<typeof canvasStore.useCanvasStore>)
   })