refactor(date-picker): idiomatic Tailwind focus ring + snapshot tests

Align with the add-component skill:
- Replace the inline JS focus-ring handlers (el.style.outline / chromeFocusProps)
  with the same `ring` utility Button/inputs use — focus-visible:ring on the nav
  buttons + trigger, and :focus:ring (with relative/z-10 so it sits above
  adjacent cells) on calendar day cells. No more CSS-in-JS.
- Add structural snapshot tests for DateField, DatePicker (closed), and
  Calendar, per the skill's testing convention.

The earlier "no visible focus" was a wrong-attribute bug (data-[focus-visible],
a react-aria attr our cells never emit), not a failure of `ring`; the compiled
utilities are present in dist and identical to Button's.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: interacsean