An out-of-range or infinite numeric resetsAt passed validation for the
stored resetsAt string (kept empty) but was kept raw in resetValue, so
presentUsageWindow rendered it literally: quota rows showed
"8640000000000001" or "Infinity" as the reset label. Both fields now
share a usableEpochMs check, and unusable numbers keep the empty reset.

Regression: test_numericResetDateStaysParsable asserts resetValue and
the presented reset text stay empty for NaN, Infinity, and
8640000000000001. Fails before, passes after.

Co-authored-by: Claude <noreply@anthropic.com>