Icon-only square button for toolbars and row actions; `label` is required and doubles as the tooltip.

```jsx
<IconButton icon="ellipsis" label="More actions" />
<IconButton icon="refresh-cw" label="Reconcile now" variant="outline" size="sm" />
```

Variants: `ghost` (default), `outline`. Sizes `md` 34px / `sm` 28px.
