## Implementation 001: Fix TypeScript type error in FocusTimer component

**Side:** frontend
**Ticket:** ticket-001
**Contract:** timer-ui v1 (no API changes)
**Ready for review:** yes

**Approach:**

Added 'completing' state to the TimerState union type. This state is used as a transient step between when the timer reaches 0 and when the backend responds to the logSession() request. Without this type, TypeScript compilation fails.

**UI States Implemented:**
- idle
- running
- paused
- completing
- completed
- error

**Client State:**

timerState manages the UI state; 'completing' is a transient state that triggers handleLogCompletion() and then transitions to 'completed' or 'error' based on the backend response

**Files:**
- frontend/src/FocusTimer.tsx: TimerState type union now includes 'completing' (line 25)
