Add month/year header dropdowns for Calendar navigation

[Copilot]

This change replaces the static month/year header text with interactive dropdowns so users can directly jump to a target month and year. The new controls are class-addressable to support the same styling model used across the rest of the component.

• Header navigation controls

  • Replaced the header label with two selects:
    • Month: .preachjs-calendar--header-month
    • Year: .preachjs-calendar--header-year
  • Kept previous/next arrow behavior unchanged.

• Date transition correctness

  • Added shared rollover-safe date update logic for dropdown changes.
  • Handles invalid day rollover when switching month/year (e.g., 31st → shorter month, leap-year transitions) by clamping to valid day-of-month.

• Styling surface

  • Added header selector container/select styling in the example app to demonstrate how month/year dropdowns can be themed consistently with existing calendar styles.

• Coverage update

  • Added focused E2E coverage for selecting month and year from dropdowns and asserting the rendered calendar context updates accordingly.

<select
  class="preachjs-calendar--header-month"
  value={String(monthInView)}
  onChange={e => {
    activeDate$.value = updateDateFromParts(activeDate$.value, {
      month: +e.target.value,
    })
  }}
>
  {monthOptions.map(month => (
    <option value={String(month.value)}>{month.label}</option>
  ))}
</select>

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