feat: add Group Rectangle — resizable decorative zones on canvas

- New GroupRectNode: rounded rect with NodeResizer (8 handles), no
  connection handles, always behind network nodes via negative zIndex
- GroupRectModal: label, font preset (Inter/Mono/Serif), 3×3 text
  position grid, text/border/background color pickers, z-order 1–9
- Sidebar: "Add Rectangle" button (below Add Node), group rects
  excluded from node count stats
- Save/load: size persisted in custom_colors.width/height, no backend
  schema changes required
- elevateNodesOnSelect=false on ReactFlow so selected rects never
  pop above network nodes
- Tests: 3 new store tests + 9 GroupRectModal tests (66 total, all pass)

frontend/src/stores/canvasStore.ts

@@ -28,6 +28,9 @@ interface CanvasState {
   updateEdge: (id: string, data: Partial<EdgeData>) => void
   deleteEdge: (id: string) => void
   setProxmoxContainerMode: (proxmoxId: string, enabled: boolean) => void
+  setNodeZIndex: (id: string, zIndex: number) => void
+  editingGroupRectId: string | null
+  setEditingGroupRectId: (id: string | null) => void
   markSaved: () => void
   loadCanvas: (nodes: Node<NodeData>[], edges: Edge<EdgeData>[]) => void
   notifyScanDeviceFound: () => void
@@ -38,6 +41,7 @@ export const useCanvasStore = create<CanvasState>((set) => ({
   edges: [],
   hasUnsavedChanges: false,
   selectedNodeId: null,
+  editingGroupRectId: null,
   scanEventTs: 0,
 
   onNodesChange: (changes) =>
@@ -141,6 +145,14 @@ export const useCanvasStore = create<CanvasState>((set) => ({
       return { nodes, hasUnsavedChanges: true }
     }),
 
+  setNodeZIndex: (id, zIndex) =>
+    set((state) => ({
+      nodes: state.nodes.map((n) => n.id === id ? { ...n, zIndex } : n),
+      hasUnsavedChanges: true,
+    })),
+
+  setEditingGroupRectId: (id) => set({ editingGroupRectId: id }),
+
   markSaved: () => set({ hasUnsavedChanges: false }),
 
   notifyScanDeviceFound: () => set({ scanEventTs: Date.now() }),