feat: 移动端/桌面端自动适配

- utils/index.ts: 新增 isMobile() 检测 user-agent
- router: beforeEach 守卫中根据设备自动跳转
  - 手机访问 / 桌面路由 → 自动跳 /m
  - 桌面访问 /m 移动路由 → 自动跳 /

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/router/index.ts

@@ -1,5 +1,6 @@
 import { createRouter, createWebHistory } from 'vue-router'
 import { useAuthStore } from '@/stores/auth'
+import { isMobile } from '@/utils'
 
 const router = createRouter({
   history: createWebHistory(),
@@ -73,6 +74,19 @@ router.beforeEach((to, _from, next) => {
     next('/login')
     return
   }
+
+  // Auto-redirect to mobile layout for mobile devices accessing desktop routes
+  const onMobile = isMobile()
+  if (onMobile && to.path === '/' && !to.path.startsWith('/m')) {
+    next('/m')
+    return
+  }
+  // Desktop user accidentally on mobile route → redirect to desktop
+  if (!onMobile && to.path.startsWith('/m')) {
+    next('/')
+    return
+  }
+
   next()
 })