Show a negative sign when a tokenized countdown has passed

[danielchalmers]

Custom TimeSpan format strings drop the sign, so a countdown 3 days past due rendered exactly like 3 days remaining with formats such as {%d}d {%h}h or the new Digital preset.

Fix: format the magnitude and prefix - when the target has passed.

Format, target 90 min ago	Before	After
{hh}:{mm}:{ss}	01:30:00	-01:30:00
{%h}h {%m}m	1h 30m	-1h 30m
c (standard, already signed)	-01:30:00	-01:30:00 (no double sign)

• The Automatic (Humanizer) path already says "ago" and is untouched
• "Bad format" errors are not prefixed
• Pre-existing behavior, surfaced by the new one-click presets
• Note: this touches the same method as Fix automatic countdown being skewed by the UTC offset #146, so whichever merges second needs a trivial rebase

[danielchalmers]

Not currently planned