fix: 手动切换布局 — 仅首页首次加载自动适配

检测 _from.name 区分「首次加载」和「应用内导航」:
- 首次: 手机→/m, 桌面→/
- 手动点击切换按钮: 完全尊重用户选择, 不拦截

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

frontend/src/router/index.ts

@@ -75,16 +75,19 @@ router.beforeEach((to, _from, next) => {
     return
   }
 
-  // Auto-redirect: mobile devices hitting root → send to mobile home
-  // Allow manual override via ?view=desktop or ?view=mobile
+  // Auto-redirect on FIRST entry only (no previous route = fresh page load)
+  // Don't interfere with manual navigation (user clicked switch button)
   const viewOverride = to.query.view as string
-  if (!viewOverride) {
+  if (!viewOverride && !_from.name) {
     const onMobile = isMobile()
-    // Only redirect bare root — explicit paths are honored
-    if (onMobile && (to.path === '/' || to.path === '')) {
+    if (onMobile && to.path === '/') {
       next('/m')
       return
     }
+    if (!onMobile && to.path.startsWith('/m')) {
+      next('/')
+      return
+    }
   }
 
   next()