fix: follow date focus moved by assistive technology

[web-padawan]

Description

Part of #12398
Depends on #12596

Nothing synced focusedDate from DOM focus. The overlay only changes it from its own keyboard
handling and from focusDate(). When a screen reader moves focus to another date, for example
VoiceOver swiping on iOS, the arrow keys continue from the stale date, the focused part stays on
the wrong cell, and PageUp / PageDown keep the stale day of month. This is a
prerequisite for letting VoiceOver reach other months, which is not part of this PR.

• Added a date-focus event to vaadin-month-calendar, fired from a focusin listener on the grid
  with the date of the focused cell
• Made the overlay content follow date-focus by setting focusedDate and _focusedMonthDate
  • Ignored when the date already matches focusedDate, so focus set by focusDate() is a no-op
  • Ignored for dates outside min and max, the same rule the arrow keys use, while disabled
    dates inside the range are followed
• Added keyboard navigation tests for following focus, the no-op case, the range guard, the day of
  month kept for PageDown, and the input showing the focused date

Type of change

• Bugfix

How to test

1. Open dev/date-picker.html and click the field to open the calendar.
2. In the console, focus another date's button:

   const dp = document.querySelector('vaadin-date-picker');
   const cal = dp._overlayContent.calendars.find((c) => !c.hasAttribute('aria-hidden'));
   [...cal.shadowRoot.querySelectorAll('[part~=date-button]')][20].focus();

3. The focus ring moves to that date and the input shows it.
4. Press ArrowRight. Focus moves one day on from that date, not from today.
5. Press PageDown. Focus lands on the same day of month in the next month.
6. Set dp.max to a date before the one you focused, repeat step 2 with a date past max. Nothing
   changes.
7. With iOS VoiceOver, open the calendar and swipe to another date. The focus ring follows the swipe.

Note

On desktop the input already follows the focused date after arrow keys, and closing the overlay
commits the input. Focus moved by assistive technology now takes the same path. On iOS the input
is read only while the overlay is open, so a swipe never changes the committed value.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[web-padawan]

When a screen reader moves focus to another date, for example VoiceOver swiping on iOS, the arrow keys continue from the stale date

This sounds like more like an edge / artificial case (e.g. an iOS device with an external keyboard).
On desktop this isn't needed: NVDA, JAWS, VoiceOver on Safari work without it. Closing.