feat(date-field): add regional dateFormat and showDayOfWeek

[interacsean]

Closes https://github.com/tailor-inc/platform-planning/issues/1883

Demo

jpdate.mp4

What

Two opt-in additions to DateField, DatePicker and DateRangePicker:

• dateFormat: "numeric" | "regional", plus an app-wide default on <AppShell dateInputDateFormat> (namespaced so a bare dateFormat stays free for broader app-wide formatting). The component prop overrides the shell setting.
  • "numeric" is the default and is today's behaviour, so nothing changes on upgrade. The docs, JSDoc and changeset note that the default is planned to become "regional" in a future major release.
  • "regional" uses the locale's written business form wherever that form keeps the month numeric: 2025年12月19日 for ja / zh, 2025년 12월 19일 for ko.
  • Locales whose written form spells the month as a word (en, de, fr, …) stay numeric. That is detected from Intl.formatToParts, with no locale table, so one app-wide setting is safe for multi-locale apps.
• showDayOfWeek appends the locale's short day of the week where the locale places it: …19日(金) in ja, Fri, 12/19/2025 in en. It is read-only and outside the tab order. The visible span is aria-hidden; instead, the full day name is appended to the date segments' aria-valuetext (e.g. 19, Friday), so it's announced while editing.

Why

ja business screens need 年月日 and a day of the week next to dates where business days matter (e.g. closing dates). Today consumers swap a hand-built label in and out of the picker to get this; ra-support needs 137 lines for it. The issue has the details.

Notes for reviewers

• When the day of the week updates. The engine emits on every keystroke once the date is complete, so the day of the week would otherwise flash an intermediate value: typing 25 shows the 2nd for one keystroke. To prevent that, the engine tracks a pending date segment, meaning one whose first digit could still take a second. While a segment is pending, the day of the week keeps showing the date from before the entry, muted, so the field doesn't reflow. It settles on auto-advance, on segment blur, or on any non-typing commit (arrows, shortcuts, calendar pick, controlled change). Time segments never pend.
• ko-KR. ko fuses the unit onto the number (12월). regionalParts splits it into a numeric part plus a literal.
• Day-of-week time zone. The day of the week is computed from the calendar date in UTC on both sides, so the host time zone can't shift it.
• Open questions for the team (not blockers):
  • Intl outputs half-width (金); ja documents usually use full-width （金）.
  • showDayOfWeek is per-component only. It could get a shell-wide default if that's wanted.
  • The –– day-of-week placeholder sits close to the range picker's – separator.
• Out of scope, found while testing. This bug predates this branch: the DatePicker calendar opens on today's month, not on a typed value. It's being handled separately.

Testing

• Unit tests (13 new) cover:
  • the default numeric form for ja
  • regional output for ja / zh / ko / en / de
  • shell default vs. prop override
  • day-of-week placement and placeholder
  • typing 25 with no flash
  • a single digit settling on blur
  • arrow-key updates
  • the range picker
• Checked by hand in the vite-app showcase (/showcase/date-picker → "Date format + day of the week"): typing, arrow keys, and picking from the calendar.
• Local runs: fmt:check, type-check, lint, test, build, check-dts, docs:check.

🤖 Generated with Claude Code

[interacsean]

/review

[github-actions]

✅ Code Review completed successfully!

Code review complete for PR #573: reviewed the changed package surfaces, export impact, and prior review context; no unresolved High or Medium issues found, so no inline review comments were needed.

Generated by Code Review for #573

[github-actions]

Code Metrics Report

	main (94f6a5a)	#573 (44ecc98)	+/-
Coverage	88.1%	88.2%	+0.0%
Test Execution Time	2m20s	1m28s	-52s

Details

  |                     | main (94f6a5a) | #573 (44ecc98) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          88.1% |          88.2% | +0.0% |
  |   Files             |            207 |            207 |     0 |
  |   Lines             |           6176 |           6216 |   +40 |
+ |   Covered           |           5446 |           5487 |   +41 |
+ | Test Execution Time |          2m20s |          1m28s |  -52s |

Code coverage of files in pull request scope (93.7% → 94.3%, patch 100.0%)

Files	Coverage	+/-	Patch Coverage	Status
packages/core/src/components/appshell/appshell.tsx	78.3%	0.0%	-	modified
packages/core/src/components/date-field/date-field.tsx	100.0%	0.0%	100.0%	modified
packages/core/src/components/date-field/date-input-group.tsx	85.4%	+0.9%	100.0%	modified
packages/core/src/components/date-field/date-range-input-group.tsx	100.0%	0.0%	-	modified
packages/core/src/components/date-field/date-range-picker.tsx	97.7%	+0.0%	100.0%	modified
packages/core/src/components/date-field/use-date-field-state.ts	97.7%	+0.8%	100.0%	modified
packages/core/src/contexts/appshell-context.tsx	95.2%	+0.7%	100.0%	modified
packages/core/src/index.ts	0.0%	0.0%	-	modified
packages/core/tests/test-utils.tsx	100.0%	0.0%	-	modified

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Reported by octocov