Add UnixTimestamp component

[CarlosNZ]

Adds a UnixTimestamp pre-built component to @json-edit-react/components — epoch numbers (seconds or milliseconds) rendered as a readable date — implementing candidate #3 from #355. It builds directly on the swappable date-picker widget landed for DatePicker.

What it does

• Guard / unit detection (epoch.ts) — matches numbers in a plausible epoch window (years 1990–2100). The seconds and millisecond bands don't overlap, so one magnitude check is both the guard and the 'auto' seconds-vs-ms detector. The detected/forced unit is preserved on commit; force it via componentProps.unit.
• Editing — reuses the same swappable DatePickerWidgetProps picker as DatePicker. The definition's factory enables edit-mode rendering only when a DatePicker widget is passed via componentProps; with none, core's standard number editor handles edits (showOnEdit stays false).
• View — displayAs: 'number' (default) renders the ordinary number node plus a small badge (default UNIX, via badgeLabel) using the ErrorIndicator passOriginalNode pattern; displayAs: 'date' shows a formatted date (optional formatter). The factory drops passOriginalNode in 'date' mode.
• The value stays a plain JSON number end-to-end — no stringifyReplacer/parseReviver plumbing.

condition vs guard

This is the first component whose guard is a pure heuristic rather than a safety contract (any number renders fine as a date), so the README documents replacing the guard as the encouraged move when you want to target timestamps exclusively by key — including real timestamps that fall outside the default 1990–2100 band (microsecond/nanosecond epochs, historical/far-future dates), which an ANDed condition would silently reject.

Demo

Adds Unix Timestamp (seconds) and Unix Timestamp (ms) under the Custom Component Library's Date & Time section (showing auto-detection), wired with ReactDatePicker for calendar editing, plus a Show Unix as raw number? toggle driving displayAs.

Notes

• README row + dedicated subsection, and a minor changeset for @json-edit-react/components.
• Package build, tsc --noEmit, and the demo's tsc -b --noEmit all pass.
• Not yet eyeballed in-browser — badge styling across light/dark themes and the seconds/ms auto-detection are worth a manual pass (Firefox too, given editor/DnD interactions).
• Carries one unrelated one-line z-index fix on the ReactDatePicker popup.

🤖 Generated with Claude Code

[github-actions]

Bundle size impact

@json-edit-react/components

Format	Base raw	PR raw	Δ raw	Base gzip	PR gzip	Δ gzip
esm	11.91 KB	13.70 KB	🔺 +1.78 KB (+14.97%)	4.02 KB	4.56 KB	🔺 +552 B (+13.40%)
cjs	12.54 KB	14.33 KB	🔺 +1.79 KB (+14.26%)	3.87 KB	4.39 KB	🔺 +531 B (+13.40%)

_{Measured from build/index.{cjs,esm}.js. Gzip at level 9.}