CountdownTimer displays negative values after the deadline has passed

[Kingsman-99]

Description

When the deadline tracked by CountdownTimer passes, the timer continues counting downward into negative values (e.g. "-1d -3h"). After the deadline the timer should stop at zero and display a configurable "Expired" or "Time's up" label instead of a negative count.

Acceptance Criteria

• When deadline < now the timer displays "Expired" (or a expiredLabel prop value) instead of negative numbers
• The interval is cleared when the deadline passes so the component stops re-rendering
• The component correctly transitions from counting down to showing the expired state without a page reload
• Existing tests continue to pass

Context

• Target file: src/components/CountdownTimer.tsx