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/index.css

@@ -111,6 +111,15 @@
     background-color: var(--surface-card) !important;
 }
 
+/* Transparent wrapper for container node types */
+.react-flow__node-proxmox,
+.react-flow__node-group {
+    background: transparent !important;
+    border: none !important;
+    box-shadow: none !important;
+    padding: 0 !important;
+}
+
 /* Mono font utility */
 .font-mono {
     font-family: 'JetBrains Mono', monospace;