fix: 手动切换布局失效 — 仅首页自动适配

- 只对根路径 / 做手机→/m 跳转，不再拦截桌面访问 /m
- 支持 ?view=desktop 或 ?view=mobile 强制覆盖自动检测
- 手动输入的具体路径完全不受影响

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

frontend/src/router/index.ts

@@ -75,16 +75,16 @@ router.beforeEach((to, _from, next) => {
     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
+  // Auto-redirect: mobile devices hitting root → send to mobile home
+  // Allow manual override via ?view=desktop or ?view=mobile
+  const viewOverride = to.query.view as string
+  if (!viewOverride) {
+    const onMobile = isMobile()
+    // Only redirect bare root — explicit paths are honored
+    if (onMobile && (to.path === '/' || to.path === '')) {
+      next('/m')
+      return
+    }
   }
 
   next()