Square borderless hit-area for a single line icon (settings gear, close, theme toggle).

```jsx
const { IconButton } = window.DuduDesignSystem_2e0172;
<IconButton label="设置" size={32} onClick={...}>
  <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round">…</svg>
</IconButton>
```

Icon inherits `currentColor` (text-2, darkens on hover). Keep icons 16–20px inside a 28–36px button.
