feat(mobile): add device detection composable and placeholder view

frontend/src/mobile/useDevice.ts

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+// src/mobile/useDevice.ts
+// 设备检测：判断当前是否移动端，结果存 localStorage 避免重复检测
+
+const STORAGE_KEY = 'device_mode'
+
+/** UA 关键字匹配移动设备 */
+const MOBILE_UA = /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i
+
+/** 屏幕宽度阈值（含平板竖屏） */
+const MOBILE_WIDTH = 768
+
+/**
+ * 原始检测：综合 UA 和屏幕宽度判断
+ * - UA 命中移动设备关键字 → 手机
+ * - 屏宽 <= 768 → 手机
+ * - 否则 → 桌面
+ */
+function detectRaw(): 'mobile' | 'desktop' {
+  if (typeof navigator === 'undefined') return 'desktop'
+  if (MOBILE_UA.test(navigator.userAgent)) return 'mobile'
+  if (typeof window !== 'undefined' && window.innerWidth <= MOBILE_WIDTH) return 'mobile'
+  return 'desktop'
+}
+
+/** 当前设备模式（读取 localStorage，无则检测并存入） */
+export function getDeviceMode(): 'mobile' | 'desktop' {
+  if (typeof localStorage === 'undefined') return detectRaw()
+  const stored = localStorage.getItem(STORAGE_KEY)
+  if (stored === 'mobile' || stored === 'desktop') return stored
+  const detected = detectRaw()
+  localStorage.setItem(STORAGE_KEY, detected)
+  return detected
+}
+
+/** 是否移动端（路由分流用，同步函数） */
+export function isMobile(): boolean {
+  return getDeviceMode() === 'mobile'
+}
+
+/**
+ * 手动切换设备模式（设置页"切换到桌面版/手机版"用）
+ * 切换后需整页刷新以重新走路由解析
+ */
+export function setDeviceMode(mode: 'mobile' | 'desktop') {
+  localStorage.setItem(STORAGE_KEY, mode)
+}
+
+/** 重置为自动检测（登出时调用，避免下个用户沿用上个用户的偏好） */
+export function resetDeviceMode() {
+  localStorage.removeItem(STORAGE_KEY)
+}

frontend/src/views-mobile/Placeholder.vue

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+<!-- src/views-mobile/Placeholder.vue -->
+<!-- 占位页：手机端尚未实现的页面暂时显示此组件 -->
+<script setup lang="ts">
+import { useRoute } from 'vue-router'
+
+const route = useRoute()
+</script>
+
+<template>
+  <div class="placeholder-page">
+    <van-empty description="该页面手机版开发中">
+      <p class="placeholder-hint">当前页面：{{ route.name }}</p>
+    </van-empty>
+  </div>
+</template>
+
+<style scoped>
+.placeholder-page {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  min-height: 60vh;
+  padding: 24px;
+}
+
+.placeholder-hint {
+  margin-top: 8px;
+  font-size: 12px;
+  color: var(--gg-text-muted);
+}
+</style>