feat: Add skeleton loading support to table

[gethinwebster]

Add skeleton prop ({ totalRows: number }) that renders skeleton placeholder rows to fill the table while data is loading.

• skeleton.totalRows defines total visible rows (typically page size)
• Skeleton rows rendered = max(0, totalRows - items.length)
• Standard loading: items=[] + loading=true + skeleton -> all skeleton rows
• Progressive loading: items=[partial] + skeleton -> real rows + trailing skeleton
• A11y: visually-hidden row with ScreenreaderOnly loadingText before skeleton rows
• Skeleton rows use aria-hidden=true
• Includes interactive demo pages for both patterns

Description

Related links, issue #, if available: n/a

How has this been tested?

Review checklist

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

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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