Primary action control. Use `primary` for the single main action per view, `secondary` for the rest, `ghost` for inline/quiet actions, `danger` for destructive ones.

```jsx
const { Button } = window.DuduDesignSystem_2e0172;
<Button variant="primary" size="md" onClick={...}>购买时长</Button>
<Button variant="secondary" size="sm">充值</Button>
<Button variant="primary" size="lg" block>微信一键登录</Button>
```

Variants: primary / secondary (bordered) / ghost (text+tint hover) / danger. Sizes sm 28px · md 36px · lg 44px (mobile). `block` fills the row. Never put two primary buttons side by side.
