feat: 初始化智能物联网设备管理系统前后端基础项目

完成项目整体结构搭建，包含：
1. 前端Vue3+TS+Vite+Antd项目基础配置与核心依赖
2. 后端MidwayJS+TypeORM+JWT权限认证框架
3. 完整的用户、角色、权限、日志数据库表结构
4. 核心业务模块API与基础服务实现
5. 全局样式、路由、状态管理配置

client/src/composables/useTheme.ts

@@ -0,0 +1,40 @@
+import { ref, watchEffect, onMounted, computed } from 'vue'
+
+type Theme = 'light' | 'dark'
+
+export function useTheme() {
+  const theme = ref<Theme>('light')
+
+  const getPreferredTheme = (): Theme => {
+    const saved = localStorage.getItem('theme') as Theme | null
+    if (saved === 'light' || saved === 'dark') return saved
+    return window.matchMedia('(prefers-color-scheme: dark)').matches
+      ? 'dark'
+      : 'light'
+  }
+
+  const applyTheme = (t: Theme) => {
+    document.documentElement.classList.remove('light', 'dark')
+    document.documentElement.classList.add(t)
+    localStorage.setItem('theme', t)
+  }
+
+  const toggleTheme = () => {
+    theme.value = theme.value === 'light' ? 'dark' : 'light'
+  }
+
+  onMounted(() => {
+    theme.value = getPreferredTheme()
+    applyTheme(theme.value)
+  })
+
+  watchEffect(() => {
+    applyTheme(theme.value)
+  })
+
+  return {
+    theme,
+    toggleTheme,
+    isDark: computed(() => theme.value === 'dark'),
+  }
+}