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/components/modals/__tests__/GroupRectModal.test.tsx

@@ -0,0 +1,83 @@
+import { describe, it, expect, vi } from 'vitest'
+import { render, screen, fireEvent } from '@testing-library/react'
+import { GroupRectModal, type GroupRectFormData } from '../GroupRectModal'
+
+describe('GroupRectModal', () => {
+  it('renders nothing when closed', () => {
+    const { container } = render(
+      <GroupRectModal open={false} onClose={vi.fn()} onSubmit={vi.fn()} />
+    )
+    expect(container.querySelector('[role="dialog"]')).toBeNull()
+  })
+
+  it('renders form fields when open', () => {
+    render(<GroupRectModal open onClose={vi.fn()} onSubmit={vi.fn()} />)
+    expect(screen.getByPlaceholderText('Zone name…')).toBeDefined()
+    expect(screen.getByText('Add Rectangle')).toBeDefined()
+    expect(screen.getByText('Text Position')).toBeDefined()
+    expect(screen.getByText('Z-Order (1 = furthest back)')).toBeDefined()
+  })
+
+  it('renders Edit Rectangle title when provided', () => {
+    render(<GroupRectModal open onClose={vi.fn()} onSubmit={vi.fn()} title="Edit Rectangle" />)
+    expect(screen.getByText('Edit Rectangle')).toBeDefined()
+  })
+
+  it('calls onSubmit with form data on submit', () => {
+    const onSubmit = vi.fn()
+    render(<GroupRectModal open onClose={vi.fn()} onSubmit={onSubmit} />)
+    const input = screen.getByPlaceholderText('Zone name…')
+    fireEvent.change(input, { target: { value: 'DMZ' } })
+    fireEvent.click(screen.getByText('Add'))
+    expect(onSubmit).toHaveBeenCalledOnce()
+    const submitted = onSubmit.mock.calls[0][0] as GroupRectFormData
+    expect(submitted.label).toBe('DMZ')
+    expect(submitted.font).toBe('inter')
+    expect(submitted.z_order).toBe(1)
+  })
+
+  it('calls onClose when Cancel is clicked', () => {
+    const onClose = vi.fn()
+    render(<GroupRectModal open onClose={onClose} onSubmit={vi.fn()} />)
+    fireEvent.click(screen.getByText('Cancel'))
+    expect(onClose).toHaveBeenCalledOnce()
+  })
+
+  it('shows Delete button when onDelete is provided', () => {
+    const onDelete = vi.fn()
+    render(<GroupRectModal open onClose={vi.fn()} onSubmit={vi.fn()} onDelete={onDelete} />)
+    expect(screen.getByText('Delete')).toBeDefined()
+  })
+
+  it('calls onDelete and onClose when Delete is clicked', () => {
+    const onDelete = vi.fn()
+    const onClose = vi.fn()
+    render(<GroupRectModal open onClose={onClose} onSubmit={vi.fn()} onDelete={onDelete} />)
+    fireEvent.click(screen.getByText('Delete'))
+    expect(onDelete).toHaveBeenCalledOnce()
+    expect(onClose).toHaveBeenCalledOnce()
+  })
+
+  it('pre-fills form from initial prop', () => {
+    render(
+      <GroupRectModal
+        open
+        onClose={vi.fn()}
+        onSubmit={vi.fn()}
+        initial={{ label: 'Pre-filled', z_order: 5, font: 'mono' }}
+      />
+    )
+    const input = screen.getByPlaceholderText('Zone name…') as HTMLInputElement
+    expect(input.value).toBe('Pre-filled')
+  })
+
+  it('selects text position button', () => {
+    const onSubmit = vi.fn()
+    render(<GroupRectModal open onClose={vi.fn()} onSubmit={onSubmit} />)
+    // Click bottom-right (↘)
+    fireEvent.click(screen.getByTitle('bottom-right'))
+    fireEvent.click(screen.getByText('Add'))
+    const submitted = onSubmit.mock.calls[0][0] as GroupRectFormData
+    expect(submitted.text_position).toBe('bottom-right')
+  })
+})