--until alone (without --since) collapses to a 1-day window instead of [default-since, until] #18

internal/cronoclient/daterange.go::resolveDateRange defaults to last-7-days only when both flags are empty:

if since.IsZero() && until.IsZero() {
    return DateRange{Start: today.AddDate(0, 0, -6), End: today}, nil
}
if until.IsZero() { until = today }
if since.IsZero() { since = until }   // <-- here

So --until 2026-04-20 (without --since) silently turns into a single-day query for 2026-04-20, not "the last 7 days ending 2026-04-20". This is surprising:

• The no-flag default is a 7-day window.
• Setting only --since extends the window to today (correct).
• Setting only --until should symmetrically extend the window 7 days back from until, but instead shrinks it to a single day.

Repro:

$ /tmp/crono-export biometrics --until 7d
_(no records in window)_     # this is a 1-day query for the day 7 days ago, not a 7-day window

Expected: --until X alone -> [X-6d, X] (mirror of the default 7-day window). Document this in prime and in the flag help.

Severity: minor