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/_layout.tsx

@@ -1,9 +1,15 @@
+import { useEffect } from 'react';
 import { DarkTheme, DefaultTheme, ThemeProvider } from '@react-navigation/native';
 import { Stack } from 'expo-router';
 import { StatusBar } from 'expo-status-bar';
+import * as SplashScreen from 'expo-splash-screen';
 import 'react-native-reanimated';
 
 import { useColorScheme } from '@/hooks/use-color-scheme';
+import { useIsOnboardingComplete } from '@/src/features/onboarding/hooks/useOnboarding';
+
+// Prevent splash screen from auto-hiding
+SplashScreen.preventAutoHideAsync();
 
 export const unstable_settings = {
   anchor: '(tabs)',
@@ -11,10 +17,32 @@ export const unstable_settings = {
 
 export default function RootLayout() {
   const colorScheme = useColorScheme();
+  const isOnboardingComplete = useIsOnboardingComplete();
+
+  // Hide splash screen once we have a definite state
+  useEffect(() => {
+    if (isOnboardingComplete !== undefined) {
+      SplashScreen.hideAsync();
+    }
+  }, [isOnboardingComplete]);
+
+  // Show nothing while Zustand hydrates from AsyncStorage
+  if (isOnboardingComplete === undefined) {
+    return null;
+  }
 
   return (
     <ThemeProvider value={colorScheme === 'dark' ? DarkTheme : DefaultTheme}>
       <Stack>
+        <Stack.Screen
+          name="onboarding"
+          options={{
+            headerShown: false,
+            presentation: 'fullScreenModal',
+            animation: 'fade',
+            gestureEnabled: false,
+          }}
+        />
         <Stack.Screen name="(tabs)" options={{ headerShown: false }} />
         <Stack.Screen
           name="timer"