feat: add formatDateDisplay prop for custom date formatting

[pataar]

Summary

• Adds a new formatDateDisplay prop that accepts (date: Date) => string for custom input display formatting
• Unlike overloading dateFormat, this preserves typed input parsing — dateFormat still handles parsing while formatDateDisplay only controls the displayed value
• Works with single dates, date ranges, multiple dates, and the formatMultipleDates callback
• Includes docs-site example demonstrating Intl.DateTimeFormat usage

Example usage

const formatter = new Intl.DateTimeFormat(undefined, {
  weekday: "short",
  year: "numeric",
  month: "long",
  day: "numeric",
});

<DatePicker
  formatDateDisplay={(date) => formatter.format(date)}
  selected={selectedDate}
  onChange={setSelectedDate}
/>

Test plan

• Unit tests for safeMultipleDatesFormat with formatDateDisplay
• Integration tests for single date, range, and partial range with formatDateDisplay
• Integration test verifying typed input parsing still works when formatDateDisplay is set
• Integration tests for formatMultipleDates callback with formatDateDisplay
• Full test suite passes (1495 tests)
• TypeScript type-check passes
• ESLint + Stylelint + Prettier pass