Underline tab strip for switching sibling views; active tab gets a 2px spruce underline.

```jsx
<Tabs
  items={[
    { id: 'board', label: 'Board', icon: 'square-kanban', count: 24 },
    { id: 'gantt', label: 'Gantt', icon: 'chart-no-axes-gantt' },
    { id: 'deps', label: 'Dependencies', icon: 'network' },
  ]}
  active={view}
  onChange={setView}
/>
```
