feat: Add rowHeight prop to Board for customizable row height

[ernst-dev]

Description

Adds an opt-in, backward-compatible rowHeight prop to Board that lets consumers override the default grid row height. This addresses the feature request in AWSUI-61568 ("Customizable board row size" / ROWSPAN_HEIGHT customization).

Motivation: the board row height is currently fixed per density (96px comfortable / 76px compact), and the minimum item height is 2 rows (≈192px). Data-light widgets are therefore forced to reserve a lot of vertical white space, and customers have resorted to patching internal CSS (which breaks the Cloudscape contract). This prop gives first-class, finer-grained vertical sizing — e.g. rowHeight={32} yields 3× the sizing precision and lets a 2-row widget be 64px instead of 192px.

What changed:

• New BoardProps.rowHeight?: number (px). When unset, the existing density-based defaults are used, so current boards render identically.
• The value flows Board → InternalBoard → internal Grid, where it drives both the pixel math (getHeight) used for layout/resize and the rendered grid-auto-rows via a --awsui-board-row-height CSS custom property — keeping layout math and rendering in sync.
• Invalid values (non-positive or non-finite) are ignored and fall back to the density default.

This reuses the existing Board/Grid layout pipeline; no changes to minRowSpan/layout semantics.

Related links, issue #, if available: AWSUI-61568

Note: the ticket is a raw feature request without a finalized API spec (it only references ROWSPAN_HEIGHT customization and a Quip doc). This PR implements a rowHeight prop as the opt-in, contract-friendly surface for that request. Happy to rename/rescope (e.g. per-density object, or a BoardItem-level minimum) during review.

How has this been tested?

• npm run build, eslint, stylelint, and tsc (unit + e2e project typecheck) all pass.
• Unit tests (npm run test:unit) pass — 330 tests, 34 files. New coverage added:
  • src/internal/grid/__tests__/grid.test.tsx — default row height, custom override, and invalid-value fallback (0/negative/NaN/Infinity).
  • src/board/__tests__/board-row-height.test.tsx — prop is forwarded to the grid, default is preserved when unset, non-positive values fall back, and the prop does not leak onto the root DOM element.
• API-docs snapshot regenerated to include the new prop.
• New dev page pages/board/row-height.page.tsx renders a board with buttons to switch between default / 32 / 48 / 64 / 96px row heights for manual/visual verification.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates. ✅ Prop documented in BoardProps; API-docs snapshot updated.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md. ✅ Opt-in; defaults unchanged.
• Changes do not include unsupported browser features, see CONTRIBUTING.md. ✅ Uses a standard CSS custom property with a static fallback.
• Changes were manually tested for accessibility, see accessibility guidelines. ⚠️ No a11y-affecting changes (layout sizing only); dev page provided for manual verification.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function. N/A — no URL handling.

Testing

• Changes are covered with new/existing unit tests? ✅ New grid + board unit tests.
• Changes are covered with new/existing integration tests? ➖ Dev page added; visual/integration coverage can be added in review if desired.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.46%. Comparing base (f388073) to head (8368df1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #447      +/-   ##
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+ Coverage   91.44%   91.46%   +0.01%     
==========================================
  Files          59       59              
  Lines        1730     1734       +4     
  Branches      467      469       +2     
==========================================
+ Hits         1582     1586       +4     
  Misses        148      148              

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