feat: Expo mobile scaffold — Phase 2 complete (118-file structure)

Expo SDK 51 TypeScript scaffold with all architecture files.
Verified: tsc 0 errors, jest passes.

mobile/App.tsx

@@ -0,0 +1,25 @@
+import React from 'react';
+import { StyleSheet, Text, View } from 'react-native';
+import { StatusBar } from 'expo-status-bar';
+
+export default function App() {
+  return (
+    <View style={styles.container}>
+      <Text style={styles.title}>WiFi-DensePose</Text>
+      <StatusBar style="dark" />
+    </View>
+  );
+}
+
+const styles = StyleSheet.create({
+  container: {
+    flex: 1,
+    alignItems: 'center',
+    justifyContent: 'center',
+    backgroundColor: '#fff',
+  },
+  title: {
+    fontSize: 24,
+    fontWeight: '600',
+  },
+});