The dashed neutral line for "the rate needed to hit the target by
the target date" was rendered on the chart but absent from the
legend. Adds a third legend chip ("Required") that renders only
when the line itself renders — i.e. when target_date is set,
monthly_target_amount is positive, and there's still ground to
cover (remaining_amount > 0). Stroke matches the JS:
`text-secondary` currentColor, dasharray 2/4, 0.5 opacity.