feat(button): add render prop support for polymorphic composition

[mattrothenberg]

Summary

Add render prop to Button component for polymorphic composition via Base UI's useRender hook. This enables rendering Button as anchors, React Router Links, Next.js Links, or any custom component.

Changes

• Button component: Add render prop using Base UI's useRender and mergeProps
• ButtonState type: New exported type for render callback consumers
• Tests: Comprehensive tests for render prop patterns
• Docs: New "Composition" section with examples for React Router, Next.js, and anchors

Usage

// As an anchor
<Button render={<a href="/about" />}>About</Button>

// With React Router
<Button render={<Link to="/dashboard" />}>Dashboard</Button>

// With Next.js
<Button render={<NextLink href="/page" />}>Page</Button>

// Render callback for state access
<Button
  loading={isLoading}
  render={(props, state) => (
    <a {...props} href="/link">
      {state.loading ? "Loading..." : "Click"}
    </a>
  )}
/>

References

• Base UI Composition Guide
• Base UI useRender Hook