feat(data-table): add custom header renderer

[IzumiSy]

Motivation

DataTable currently only exposes label for column headers, which makes common customizations like icons, partial text styling, or sort-aware header affordances awkward or impossible without forking the built-in header renderer.

This change adds a small header customization API that covers those use cases while preserving the existing built-in table header structure, sticky behavior, sort click handling, and sort indicator rendering.

Design Decision

Chosen approach

Add a header field to Column that accepts either a static ReactNode or a render function.

The render function receives a narrow HeaderRenderContext instead of the whole table/control state. That keeps the API focused on header rendering and avoids coupling column definitions to internal sort policy details.

Sort interaction

Sortable headers receive { label, sortable: true, sortDirection, activateSort }.

Non-sortable headers receive { label, sortable: false }.

This keeps the type surface explicit: custom headers can read sort state and trigger the built-in sort action when sorting is actually active, but they do not need to reimplement the sort direction cycle themselves.

Alternatives considered

A full header-cell renderer or raw sort-control API would be more flexible, but it would also make it easier to break shared behavior such as sticky headers, click targets, sort indicators, and accessibility. The smaller header API is enough for the current use cases without opening that larger escape hatch.

Summary

• add Column.header for static or render-function-based custom header content
• expose a discriminated HeaderRenderContext with sort-aware data and activateSort() only when sorting is active
• keep built-in header cell layout and sort handling intact, and add tests plus a changeset for the new API

[IzumiSy]

/review

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