feat: onboarding flow (6 screens) + audio engine + design system

Onboarding:
- 6-screen flow: Problem → Empathy → Solution → Wow → Personalization → Paywall
- useOnboarding hook with Zustand + AsyncStorage persistence
- MiniTimerDemo with live 20s timer + haptics
- Auto-redirect for first-time users
- Mock RevenueCat for dev testing

Audio:
- useAudioEngine hook with expo-av
- Phase sounds (count_3/2/1, beep, bell, fanfare)
- Placeholder music tracks

Design System:
- Typography component + constants
- GlassView component
- Spacing, shadows, animations, borderRadius constants
- Extended color palette (phase gradients, glass, surfaces)

Timer:
- Fix: handle 0-duration phases (immediate advance)
- Enhanced TimerDisplay with phase gradients

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/timer.tsx

@@ -1,10 +1,73 @@
+import { useEffect } from 'react'
 import { useRouter } from 'expo-router'
+import * as Haptics from 'expo-haptics'
 import { useTimerEngine } from '@/src/features/timer'
+import { useAudioEngine } from '@/src/features/audio'
 import { TimerDisplay } from '@/src/features/timer/components/TimerDisplay'
+import type { TimerEvent } from '@/src/features/timer/types'
 
 export default function TimerScreen() {
   const router = useRouter()
   const timer = useTimerEngine()
+  const audio = useAudioEngine()
+
+  // Preload audio on mount
+  useEffect(() => {
+    audio.preloadAll()
+    return () => {
+      audio.unloadAll()
+    }
+  }, [])
+
+  // Subscribe to timer events → trigger audio + haptics
+  useEffect(() => {
+    const unsubscribe = timer.addEventListener(async (event: TimerEvent) => {
+      switch (event.type) {
+        case 'PHASE_CHANGED':
+          await handlePhaseChange(event.to)
+          break
+
+        case 'COUNTDOWN_TICK':
+          await audio.playPhaseSound(
+            event.secondsLeft === 1 ? 'count_1' : event.secondsLeft === 2 ? 'count_2' : 'count_3'
+          )
+          Haptics.impactAsync(Haptics.ImpactFeedbackStyle.Light)
+          break
+
+        case 'ROUND_COMPLETED':
+          await audio.playPhaseSound('bell')
+          Haptics.notificationAsync(Haptics.NotificationFeedbackType.Success)
+          break
+
+        case 'SESSION_COMPLETE':
+          await audio.playPhaseSound('fanfare')
+          await audio.stopMusic(1000)
+          Haptics.notificationAsync(Haptics.NotificationFeedbackType.Success)
+          break
+      }
+    })
+
+    return unsubscribe
+  }, [audio.isLoaded])
+
+  async function handlePhaseChange(to: string) {
+    switch (to) {
+      case 'GET_READY':
+        await audio.startMusic('LOW')
+        Haptics.impactAsync(Haptics.ImpactFeedbackStyle.Medium)
+        break
+      case 'WORK':
+        await audio.playPhaseSound('beep_long')
+        await audio.switchIntensity('HIGH')
+        Haptics.impactAsync(Haptics.ImpactFeedbackStyle.Heavy)
+        break
+      case 'REST':
+        await audio.playPhaseSound('beep_double')
+        await audio.switchIntensity('LOW')
+        Haptics.impactAsync(Haptics.ImpactFeedbackStyle.Light)
+        break
+    }
+  }
 
   function handleStart() {
     timer.start()
@@ -12,6 +75,7 @@ export default function TimerScreen() {
 
   function handleStop() {
     timer.stop()
+    audio.stopMusic(300)
     router.back()
   }