[UX] Deadline countdown: show days/hours/minutes/seconds format

## Problem
Deadline shows raw hours: `Next plot due in 161:48:23` — hard to parse.

## Expected
`Next plot due in 6d 23h 13m 21s`

## Format rules
- 1+ days: `Xd Xh Xm Xs` (e.g. "6d 23h 13m 21s")
- Under 24h: `Xh Xm Xs`
- Under 1h: `Xm Xs`

## Files
- `src/components/DeadlineCountdown.tsx`