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/__tests__/canvasStore.test.ts

@@ -25,6 +25,7 @@ describe('canvasStore', () => {
       edges: [],
       hasUnsavedChanges: false,
       selectedNodeId: null,
+      editingGroupRectId: null,
     })
   })
 
@@ -190,6 +191,40 @@ describe('canvasStore', () => {
     expect(updatedChild?.extent).toBeUndefined()
   })
 
+  it('setEditingGroupRectId sets and clears the editing id', () => {
+    useCanvasStore.getState().setEditingGroupRectId('rect-1')
+    expect(useCanvasStore.getState().editingGroupRectId).toBe('rect-1')
+    useCanvasStore.getState().setEditingGroupRectId(null)
+    expect(useCanvasStore.getState().editingGroupRectId).toBeNull()
+  })
+
+  it('setNodeZIndex updates the node zIndex and marks unsaved', () => {
+    useCanvasStore.getState().addNode(makeNode('n1'))
+    useCanvasStore.getState().markSaved()
+    useCanvasStore.getState().setNodeZIndex('n1', -5)
+    const node = useCanvasStore.getState().nodes.find((n) => n.id === 'n1')
+    expect(node?.zIndex).toBe(-5)
+    expect(useCanvasStore.getState().hasUnsavedChanges).toBe(true)
+  })
+
+  it('addNode with groupRect type preserves zIndex and dimensions', () => {
+    const rectNode: Node<NodeData> = {
+      id: 'rect-1',
+      type: 'groupRect',
+      position: { x: 100, y: 100 },
+      data: { label: 'Zone A', type: 'groupRect', status: 'unknown', services: [] },
+      width: 360,
+      height: 240,
+      zIndex: -9,
+    }
+    useCanvasStore.getState().addNode(rectNode)
+    const stored = useCanvasStore.getState().nodes.find((n) => n.id === 'rect-1')
+    expect(stored?.type).toBe('groupRect')
+    expect(stored?.zIndex).toBe(-9)
+    expect(stored?.width).toBe(360)
+    expect(stored?.height).toBe(240)
+  })
+
   it('loadCanvas sorts parents before children', () => {
     const parent = makeNode('p1')
     const child: Node<NodeData> = { ...makeNode('c1', { parent_id: 'p1' }), parentId: 'p1', extent: 'parent' }