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/utils/index.ts

@@ -3,3 +3,9 @@ export function todayStr(): string {
   const d = new Date()
   return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`
 }
+
+/** Detect if current device is mobile based on user-agent. */
+export function isMobile(): boolean {
+  const ua = navigator.userAgent || ''
+  return /Android|webOS|iPhone|iPad|iPod|BlackBerry|Windows Phone/i.test(ua)
+}