修复头部用户菜单关闭后按钮残留高亮，优化 hover 与 focus 样式。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/layouts/MainLayout.tsx

@@ -175,7 +175,11 @@ export default function MainLayout() {
                         : <span className="header-user-initial">{userInitial}</span>}
                     </button>
                   </DropdownMenuTrigger>
-                  <DropdownMenuContent align="end" className="w-40">
+                  <DropdownMenuContent
+                    align="end"
+                    className="w-40"
+                    onCloseAutoFocus={(e) => e.preventDefault()}
+                  >
                     <DropdownMenuItem onClick={() => nav('/profile')}>个人中心</DropdownMenuItem>
                     <DropdownMenuItem onClick={() => nav('/favorites')}>我的收藏</DropdownMenuItem>
                     {user.role === 'admin' && (

frontend/src/styles/global.css

@@ -355,14 +355,20 @@ a:hover { text-decoration: underline; }
   transition: box-shadow 0.15s, transform 0.1s;
 }
 
-.header-user-btn:hover,
+@media (hover: hover) {
+  .header-user-btn:hover {
+    box-shadow: 0 0 0 2px var(--j13-green-bg), 0 0 0 3px var(--j13-green);
+  }
+}
+
 .header-user-btn:focus-visible {
   box-shadow: 0 0 0 2px var(--j13-green-bg), 0 0 0 3px var(--j13-green);
 }
 
-/* 鼠标点击打开/关闭下拉后 focus 会回到按钮，隐藏默认黑色 outline */
-.header-user-btn:focus {
+/* 鼠标点击打开/关闭下拉后 focus 会回到按钮，隐藏默认 outline 与 hover 同款高亮 */
+.header-user-btn:focus:not(:focus-visible) {
   outline: none;
+  box-shadow: none;
 }
 
 .header-user-avatar {