fix(ui): gate app sidebar to md+ and use onSelect for logout (keyboard path)

frontend/src/components/app-shell.tsx

@@ -57,10 +57,8 @@ export function AppShell() {
       <DropdownMenuContent align="end">
         <DropdownMenuLabel>{user?.username}{isAdmin ? " · admin" : ""}</DropdownMenuLabel>
         <DropdownMenuSeparator />
-        <DropdownMenuItem asChild>
-          <span className="cursor-pointer" onClick={logout}>
-            <LogOut /> 退出登录
-          </span>
+        <DropdownMenuItem onSelect={logout}>
+          <LogOut /> 退出登录
         </DropdownMenuItem>
       </DropdownMenuContent>
     </DropdownMenu>
@@ -69,7 +67,7 @@ export function AppShell() {
   return (
     <div className="flex h-full">
       {/* 桌面/平板侧栏;lg 全文标签,md 图标栏 */}
-      <aside className="flex w-14 shrink-0 flex-col gap-1 border-r bg-card/60 p-2 lg:w-60">
+      <aside className="hidden w-14 shrink-0 flex-col gap-1 border-r bg-card/60 p-2 md:flex lg:w-60">
         <Brand compact />
         <nav aria-label="主导航" className="flex flex-col gap-1">
           <NavItem to="/" icon={<BookOpen className="size-4 shrink-0" />} label="书架" />

frontend/src/components/theme-toggle.tsx

@@ -14,7 +14,7 @@ export function ThemeToggle() {
   return (
     <DropdownMenu>
       <DropdownMenuTrigger asChild>
-        <Button variant="ghost" size="icon" aria-label="切换主题" aria-pressed="false">
+        <Button variant="ghost" size="icon" aria-label="切换主题">
           {theme === "dark" ? <Moon className="size-4" /> : theme === "light" ? <Sun className="size-4" /> : <Monitor className="size-4" />}
         </Button>
       </DropdownMenuTrigger>