feat(Calendar): week number

[maxffarrell]

Adds a `showWeekNumbers` prop to `Calendar`, `RangeCalendar`, `DatePicker`, and `DateRangePicker`, along with two new primitives — `WeekNumberCell` and `WeekNumberHeadCell` — for rendering ISO 8601 week numbers in the calendar grid.

Week numbers use Thursday-anchored ISO 8601 calculation and include a leading zero for single-digit weeks.

New components

• Calendar.WeekNumberCell / RangeCalendar.WeekNumberCell — renders the week number for a given row
• Calendar.WeekNumberHeadCell / RangeCalendar.WeekNumberHeadCell — renders the column header for the week number column

Usage

<Calendar.Root showWeekNumbers>
  {#snippet children({ months, weekdays })}
    <Calendar.Header>...</Calendar.Header>
    {#each months as month}
      <Calendar.Grid>
        <Calendar.GridHead>
          <Calendar.GridRow>
            <Calendar.WeekNumberHeadCell />
            {#each weekdays as day}
              <Calendar.HeadCell>{day}</Calendar.HeadCell>
            {/each}
          </Calendar.GridRow>
        </Calendar.GridHead>
        <Calendar.GridBody>
          {#each month.weeks as weekDates}
            <Calendar.GridRow>
              <Calendar.WeekNumberCell {weekDates} />
              {#each weekDates as date}
                <Calendar.Cell {date} month={month.value}>
                  <Calendar.Day />
                </Calendar.Cell>
              {/each}
            </Calendar.GridRow>
          {/each}
        </Calendar.GridBody>
      </Calendar.Grid>
    {/each}
  {/snippet}
</Calendar.Root>

Docs and browser tests included.

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