feat: add connection handles to zone nodes (closes #58)

- GroupRectNode now renders source+target handles on all four sides
  (top, right, bottom, left) using IDs zone-{side} / zone-{side}-t
- Handles are hover-only: opacity 0 by default, fade in on mouse enter
- Handle color matches the zone border color (respects custom_colors)
- Zone↔zone and zone↔node connections both allowed; edge type picker
  (EdgeModal) opens on connect so user chooses ethernet/wifi/vlan/etc.
- Add GroupRectNode.test.tsx: verifies 8 handles rendered (4 source + 4 target)
- Fix @xyflow/react mocks in LiveView and CanvasContainer tests to include Position

frontend/src/components/__tests__/LiveView.test.tsx

@@ -11,6 +11,7 @@ vi.mock('@xyflow/react', () => ({
   Controls: () => null,
   BackgroundVariant: { Dots: 'dots' },
   ConnectionMode: { Loose: 'loose' },
+  Position: { Top: 'top', Right: 'right', Bottom: 'bottom', Left: 'left' },
   useReactFlow: () => ({ fitView: vi.fn() }),
 }))
 vi.mock('@xyflow/react/dist/style.css', () => ({}))
@@ -143,6 +144,7 @@ const XYFLOW_MOCK = {
   Controls: () => null,
   BackgroundVariant: { Dots: 'dots' },
   ConnectionMode: { Loose: 'loose' },
+  Position: { Top: 'top', Right: 'right', Bottom: 'bottom', Left: 'left' },
   useReactFlow: () => ({ fitView: vi.fn() }),
 }