Add date/time utilities for cycle and deadline calculations

Create helpers for converting between timestamps and human-readable dates.

**Acceptance Criteria:**
- Add `formatDeadline(timestamp)` returning 'in 2 days, 5 hours'
- Add `calculateNextDeadline(currentDeadline, cycleLength)`
- Add `isDeadlinePassed(deadline)` boolean check
- Handle timezone-agnostic UNIX timestamps

**Files:** `src/utils.ts`