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/onboarding/index.tsx

@@ -0,0 +1,49 @@
+import { useRouter, Redirect } from 'expo-router'
+import { Screen1Problem } from '@/src/features/onboarding/screens/Screen1Problem'
+import { Screen2Empathy } from '@/src/features/onboarding/screens/Screen2Empathy'
+import { Screen3Solution } from '@/src/features/onboarding/screens/Screen3Solution'
+import { Screen4WowMoment } from '@/src/features/onboarding/screens/Screen4WowMoment'
+import { Screen5Personalization } from '@/src/features/onboarding/screens/Screen5Personalization'
+import { Screen6Paywall } from '@/src/features/onboarding/screens/Screen6Paywall'
+import { useOnboarding } from '@/src/features/onboarding/hooks/useOnboarding'
+
+export default function OnboardingRouter() {
+  const router = useRouter()
+  const currentStep = useOnboarding((state) => state.currentStep)
+  const isOnboardingComplete = useOnboarding((state) => state.isOnboardingComplete)
+  const nextStep = useOnboarding((state) => state.nextStep)
+  const completeOnboarding = useOnboarding((state) => state.completeOnboarding)
+
+  const handleNext = () => {
+    nextStep()
+  }
+
+  const handleComplete = () => {
+    completeOnboarding()
+    router.replace('/(tabs)')
+  }
+
+  // Redirect to tabs if onboarding is already complete
+  if (isOnboardingComplete) {
+    return <Redirect href="/(tabs)" />
+  }
+
+  // Render the correct screen based on current step
+  switch (currentStep) {
+    case 0:
+      return <Screen1Problem onNext={handleNext} />
+    case 1:
+      return <Screen2Empathy onNext={handleNext} />
+    case 2:
+      return <Screen3Solution onNext={handleNext} />
+    case 3:
+      return <Screen4WowMoment />
+    case 4:
+      return <Screen5Personalization onNext={handleNext} />
+    case 5:
+      return <Screen6Paywall onComplete={handleComplete} />
+    default:
+      // Fallback to first screen if step is out of bounds
+      return <Screen1Problem onNext={handleNext} />
+  }
+}