feat(布局): 为个人信息菜单项添加图标并简化语言选项显示

为个人信息菜单项添加用户图标，提升视觉一致性
将语言选项的显示从"中文/English"简化为"CN/EN"

frontend/src/layouts/MainLayout.tsx

@@ -285,7 +285,7 @@ export default function MainLayout() {
   }, [currentAncestors])
 
   const dropdownMenuItems = useMemo(() => ([
-    { key: 'profile', label: '个人信息' },
+    { key: 'profile', label: '个人信息', icon: <UserOutlined /> },
     { type: 'divider' as any },
     { key: 'logout', label: '退出登录', icon: <LogoutOutlined /> },
   ]), [])
@@ -505,8 +505,8 @@ export default function MainLayout() {
                <Dropdown
                  menu={{
                    items: [
-                     { key: 'zh', label: '中文' },
-                     { key: 'en', label: 'English' },
+                     { key: 'zh', label: 'CN' },
+                     { key: 'en', label: 'EN' },
                    ],
                    onClick: ({ key }) => { const v = key as 'zh'|'en'; setLang(v); localStorage.setItem('lang', v) }
                  }}
@@ -514,7 +514,7 @@ export default function MainLayout() {
                >
                  <Space style={{ cursor: 'pointer', userSelect: 'none' }}>
                    <GlobalOutlined />
-                   <span>{lang === 'zh' ? '中文' : 'English'}</span>
+                   <span>{lang === 'zh' ? 'CN' : 'EN'}</span>
                  </Space>
                </Dropdown>
                <Dropdown