feat: timer engine + full-screen timer UI
Implement the core timer feature following the src/features/ architecture:
- useTimerEngine hook: drift-free Date.now() delta countdown (100ms tick),
explicit state machine (IDLE → GET_READY → WORK → REST → COMPLETE),
event emitter for external consumers (PHASE_CHANGED, ROUND_COMPLETED,
COUNTDOWN_TICK, SESSION_COMPLETE), auto-pause on AppState interruption
(phone calls, background), expo-keep-awake during session
- TimerDisplay component: full-screen animated UI with 600ms color
transitions between phases, pulse animation on countdown, flash red
on last 3 seconds, round progress dots, IDLE/active/COMPLETE views
- TimerControls component: stop/pause-resume/skip buttons with Ionicons
- Timer route (app/timer.tsx): fullScreenModal wiring hook → display
- Home screen: dark theme with START button navigating to /timer
- Project docs: CLAUDE.md (constitution), PRD v1.1, skill files
- Shared constants: PHASE_COLORS, TIMER_DEFAULTS, formatTime utility
- Types: TimerPhase, TimerState, TimerConfig, TimerActions, TimerEvent
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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