feat(date-field): add showDayOfWeek

[interacsean]

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

What

Adds showDayOfWeek to DateField, DatePicker and DateRangePicker. It shows the locale's short day of the week where the locale places it:

• ja-JP: 2025/12/19(金)
• en-US: Fri, 12/19/2025

Why

Business screens need the day of the week next to dates where "is this a business day" matters, such as closing dates. Today ra-support swaps a hand-built label in and out of the picker to get it. After talking to the people who gave the feedback, the team agreed the day of the week alone covers the need. The 年月日 layout explored in #573 is not wanted for inputs, since YYYY/MM/DD is the ja-JP norm. That work stays on feat/date-field/1883-regional-date-format-weekday in case it comes back.

Notes for reviewers

• The day of the week is read-only. It is derived from the composed date and shows a –– placeholder until the date is complete.
• No flashing while typing. The engine emits on every keystroke once the date is complete, so typing 25 into the day would briefly show the 2nd's day of the week. 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 previous day of the week stays on screen, muted, so the field doesn't reflow.
  • It settles on auto-advance, on segment blur, or on any non-typing change (arrows, shortcuts, calendar pick, controlled value).
  • Time segments never pend.
• Accessibility. The visible span is aria-hidden. Instead, the full day name is appended to the year/month/day segments' aria-valuetext (e.g. 19, Friday), so it's announced while editing.
• 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 question: Intl outputs half-width (金), and ja documents often use full-width （金）.

Testing

• Unit tests cover:
  • placement per locale
  • the placeholder
  • typing 25 with no flash
  • a single digit settling on blur
  • arrow keys
  • time editing not affecting the day of the week
  • aria-valuetext
  • the range picker
• Showcase: vite-app /showcase/date-picker → "Day of the week".
• 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 #603: no High/Medium package issues found, so no inline review comment was needed. The available safe-output tools in this run did not include a general PR comment action.

Generated by Code Review for #603

[github-actions]

Code Metrics Report

	main (8d46cdf)	#603 (5cd5669)	+/-
Coverage	88.6%	88.7%	+0.0%
Test Execution Time	2m23s	2m43s	+20s

Details

  |                     | main (8d46cdf) | #603 (5cd5669) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          88.6% |          88.7% | +0.0% |
  |   Files             |            208 |            208 |     0 |
  |   Lines             |           6263 |           6285 |   +22 |
+ |   Covered           |           5552 |           5575 |   +23 |
- | Test Execution Time |          2m23s |          2m43s |  +20s |

Code coverage of files in pull request scope (94.9% → 95.3%, patch 100.0%)

Files	Coverage	+/-	Patch Coverage	Status
packages/core/src/components/date-field/date-field.tsx	100.0%	0.0%	-	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%	-	modified
packages/core/src/components/date-field/use-date-field-state.ts	97.5%	+0.7%	100.0%	modified

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Reported by octocov

[IzumiSy]

Great 👏