feat: 实现减脂体重管理App完整功能

- 实现拍照识别食物功能（集成大语言模型视觉能力）
- 实现智能对话功能（集成大语言模型流式输出）
- 实现食物记录和卡路里管理功能
- 实现体重记录和统计功能
- 实现健康数据管理页面
- 配置数据库表结构（用户、食物记录、体重记录）
- 实现Express后端API路由
- 配置Tab导航和前端页面
- 采用健康运动配色方案

client/app/(tabs)/_layout.tsx

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+import { Tabs } from 'expo-router';
+import { Platform } from 'react-native';
+import { useSafeAreaInsets } from 'react-native-safe-area-context';
+import { FontAwesome6 } from '@expo/vector-icons';
+import { useTheme } from '@/hooks/useTheme';
+
+export default function TabLayout() {
+  const { theme, isDark } = useTheme();
+  const insets = useSafeAreaInsets();
+
+  return (
+    <Tabs
+      screenOptions={{
+        headerShown: false,
+        tabBarStyle: {
+          backgroundColor: theme.backgroundDefault,
+          borderTopColor: theme.border,
+          height: Platform.OS === 'web' ? 60 : 50 + insets.bottom,
+          paddingBottom: Platform.OS === 'web' ? 0 : insets.bottom,
+        },
+        tabBarActiveTintColor: theme.primary,
+        tabBarInactiveTintColor: theme.textMuted,
+        tabBarItemStyle: {
+          height: Platform.OS === 'web' ? 60 : undefined,
+        },
+      }}
+    >
+      <Tabs.Screen
+        name="index"
+        options={{
+          title: '首页',
+          tabBarIcon: ({ color }) => <FontAwesome6 name="house" size={20} color={color} />,
+        }}
+      />
+      <Tabs.Screen
+        name="record"
+        options={{
+          title: '记录',
+          tabBarIcon: ({ color }) => <FontAwesome6 name="camera" size={20} color={color} />,
+        }}
+      />
+      <Tabs.Screen
+        name="stats"
+        options={{
+          title: '统计',
+          tabBarIcon: ({ color }) => <FontAwesome6 name="chart-line" size={20} color={color} />,
+        }}
+      />
+      <Tabs.Screen
+        name="chat"
+        options={{
+          title: 'AI助手',
+          tabBarIcon: ({ color }) => <FontAwesome6 name="robot" size={20} color={color} />,
+        }}
+      />
+    </Tabs>
+  );
+}

client/app/(tabs)/chat.tsx

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+export { default } from "@/screens/chat";

client/app/(tabs)/index.tsx

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+export { default } from "@/screens/home";

client/app/(tabs)/record.tsx

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+export { default } from "@/screens/record";

client/app/(tabs)/stats.tsx

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+export { default } from "@/screens/stats";

client/app/+not-found.tsx

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+import { View, Text, StyleSheet } from 'react-native';
+import { Link } from 'expo-router';
+import { useTheme } from '@/hooks/useTheme';
+import { Spacing } from '@/constants/theme';
+
+export default function NotFoundScreen() {
+  const { theme } = useTheme();
+
+  return (
+    <View style={[styles.container, { backgroundColor: theme.backgroundRoot }]}>
+      <Text>
+        页面不存在
+      </Text>
+      <Link href="/" style={[styles.gohome]}>
+        返回首页
+      </Link>
+    </View>
+  );
+}
+
+const styles = StyleSheet.create({
+  container: {
+    flex: 1,
+    justifyContent: 'center',
+    alignItems: 'center',
+  },
+  gohome: {
+    marginTop: Spacing['2xl'],
+  },
+});

client/app/_layout.tsx

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+import { Stack } from 'expo-router';
+
+export default function RootLayout() {
+  return (
+    <Stack screenOptions={{ headerShown: false }}>
+      <Stack.Screen name="(tabs)" />
+    </Stack>
+  );
+}