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>

app/(tabs)/index.tsx

@@ -1,98 +1,67 @@
-import { Image } from 'expo-image';
-import { Platform, StyleSheet } from 'react-native';
-
-import { HelloWave } from '@/components/hello-wave';
-import ParallaxScrollView from '@/components/parallax-scroll-view';
-import { ThemedText } from '@/components/themed-text';
-import { ThemedView } from '@/components/themed-view';
-import { Link } from 'expo-router';
+import { Pressable, StyleSheet, Text, View } from 'react-native'
+import { useRouter } from 'expo-router'
+import { StatusBar } from 'expo-status-bar'
+import { useSafeAreaInsets } from 'react-native-safe-area-context'
 
 export default function HomeScreen() {
-  return (
-    <ParallaxScrollView
-      headerBackgroundColor={{ light: '#A1CEDC', dark: '#1D3D47' }}
-      headerImage={
-        <Image
-          source={require('@/assets/images/partial-react-logo.png')}
-          style={styles.reactLogo}
-        />
-      }>
-      <ThemedView style={styles.titleContainer}>
-        <ThemedText type="title">Welcome!</ThemedText>
-        <HelloWave />
-      </ThemedView>
-      <ThemedView style={styles.stepContainer}>
-        <ThemedText type="subtitle">Step 1: Try it</ThemedText>
-        <ThemedText>
-          Edit <ThemedText type="defaultSemiBold">app/(tabs)/index.tsx</ThemedText> to see changes.
-          Press{' '}
-          <ThemedText type="defaultSemiBold">
-            {Platform.select({
-              ios: 'cmd + d',
-              android: 'cmd + m',
-              web: 'F12',
-            })}
-          </ThemedText>{' '}
-          to open developer tools.
-        </ThemedText>
-      </ThemedView>
-      <ThemedView style={styles.stepContainer}>
-        <Link href="/modal">
-          <Link.Trigger>
-            <ThemedText type="subtitle">Step 2: Explore</ThemedText>
-          </Link.Trigger>
-          <Link.Preview />
-          <Link.Menu>
-            <Link.MenuAction title="Action" icon="cube" onPress={() => alert('Action pressed')} />
-            <Link.MenuAction
-              title="Share"
-              icon="square.and.arrow.up"
-              onPress={() => alert('Share pressed')}
-            />
-            <Link.Menu title="More" icon="ellipsis">
-              <Link.MenuAction
-                title="Delete"
-                icon="trash"
-                destructive
-                onPress={() => alert('Delete pressed')}
-              />
-            </Link.Menu>
-          </Link.Menu>
-        </Link>
+  const router = useRouter()
+  const insets = useSafeAreaInsets()
 
-        <ThemedText>
-          {`Tap the Explore tab to learn more about what's included in this starter app.`}
-        </ThemedText>
-      </ThemedView>
-      <ThemedView style={styles.stepContainer}>
-        <ThemedText type="subtitle">Step 3: Get a fresh start</ThemedText>
-        <ThemedText>
-          {`When you're ready, run `}
-          <ThemedText type="defaultSemiBold">npm run reset-project</ThemedText> to get a fresh{' '}
-          <ThemedText type="defaultSemiBold">app</ThemedText> directory. This will move the current{' '}
-          <ThemedText type="defaultSemiBold">app</ThemedText> to{' '}
-          <ThemedText type="defaultSemiBold">app-example</ThemedText>.
-        </ThemedText>
-      </ThemedView>
-    </ParallaxScrollView>
-  );
+  return (
+    <View style={[styles.container, { paddingTop: insets.top + 24 }]}>
+      <StatusBar style="light" />
+
+      <Text style={styles.title}>TABATAGO</Text>
+      <Text style={styles.subtitle}>Entraînement Tabata</Text>
+
+      <Pressable
+        style={({ pressed }) => [
+          styles.startButton,
+          pressed && styles.startButtonPressed,
+        ]}
+        onPress={() => router.push('/timer')}
+      >
+        <Text style={styles.startButtonText}>START</Text>
+      </Pressable>
+    </View>
+  )
 }
 
 const styles = StyleSheet.create({
-  titleContainer: {
-    flexDirection: 'row',
+  container: {
+    flex: 1,
+    backgroundColor: '#1E1E2E',
     alignItems: 'center',
-    gap: 8,
+    justifyContent: 'center',
+    gap: 12,
   },
-  stepContainer: {
-    gap: 8,
-    marginBottom: 8,
+  title: {
+    fontSize: 44,
+    fontWeight: '900',
+    color: '#FFFFFF',
+    letterSpacing: 6,
   },
-  reactLogo: {
-    height: 178,
-    width: 290,
-    bottom: 0,
-    left: 0,
-    position: 'absolute',
+  subtitle: {
+    fontSize: 18,
+    color: 'rgba(255, 255, 255, 0.6)',
+    fontWeight: '500',
   },
-});
+  startButton: {
+    width: 160,
+    height: 160,
+    borderRadius: 80,
+    backgroundColor: '#F97316',
+    alignItems: 'center',
+    justifyContent: 'center',
+    marginTop: 60,
+  },
+  startButtonPressed: {
+    opacity: 0.7,
+  },
+  startButtonText: {
+    fontSize: 32,
+    fontWeight: '900',
+    color: '#FFFFFF',
+    letterSpacing: 4,
+  },
+})