Improvement: guard against invalid date strings in format.ts

[hydraInsurgent]

TL;DR

Malformed deadline strings produce NaN in deadlineColorClass, which silently falls back to the wrong color.

Current State

A bad date string from the API causes the deadline to appear as a far-future date (muted gray) with no indication of an error.

Desired State

Invalid dates fall back to the null state (no deadline styling) rather than a misleading color.