fix(activity): correct Year trends per-day average for the current month - #79

Bug

In the Activity Year (12-month) trends, each month's total was divided by its full calendar length. For the in-progress current month this spread a partial-month total across 30/31 days — understating the current-month steps/day bar by up to ~3× (e.g. on the 10th: total/31 instead of total/10) and mildly dragging down the Year "avg/day" headline.

Only the current month was affected; completed months were correct. Bug is specific to the .twelveMonths builder — Day/Week/Month ranges were fine.

Fix

Divide the in-progress current month by elapsed calendar days (day-of-month); completed months keep their full calendar length. This matches the existing "total ÷ calendar days in period" convention (completed months already divide by full length regardless of data gaps).

let isCurrentMonth = cal.isDate(monthStart, equalTo: Date(), toGranularity: .month)
let days = isCurrentMonth
    ? cal.component(.day, from: Date())
    : (cal.range(of: .day, in: .month, for: monthStart)?.count ?? 30)

Validation

• Completed months: else branch is byte-identical to before — no regression.
• isCurrentMonth selects exactly the one current year+month bucket (same Calendar.current throughout — no tz/DST/rollover misfire).
• Day 1 → ÷1; no div-by-zero.
• Headline (dailyAverage) and distance/calories inherit the corrected value; no other consumers affected.