feat: 实现多主题系统，支持6套主题切换和设置页面

重构 ThemeContext 为多主题模型（themeId + followSystem + systemIsDark），
新增设置页面（主题下拉栏 + 跟随系统开关），移除旧 ThemeToggle 按钮，
引入 antd-style 和 clsx 依赖支持 MUI/shadcn/Bootstrap/玻璃主题。

frontend/src/components/AppLayout/index.tsx

@@ -1,24 +1,26 @@
 import { useState } from 'react';
 import { Layout, Menu } from 'antd';
-import { CloudServerOutlined, BarChartOutlined } from '@ant-design/icons';
+import { CloudServerOutlined, BarChartOutlined, SettingOutlined } from '@ant-design/icons';
 import { Outlet, useLocation, useNavigate } from 'react-router';
-import { ThemeToggle } from '@/components/ThemeToggle';
 import { useTheme } from '@/contexts/ThemeContext';
 
 const menuItems = [
   { key: '/providers', label: '供应商管理', icon: <CloudServerOutlined /> },
   { key: '/stats', label: '用量统计', icon: <BarChartOutlined /> },
+  { type: 'divider' as const },
+  { key: '/settings', label: '设置', icon: <SettingOutlined /> },
 ];
 
 export function AppLayout() {
   const location = useLocation();
   const navigate = useNavigate();
-  const { mode } = useTheme();
+  const { effectiveThemeId } = useTheme();
   const [collapsed, setCollapsed] = useState(false);
 
   const getPageTitle = () => {
     if (location.pathname === '/providers') return '供应商管理';
     if (location.pathname === '/stats') return '用量统计';
+    if (location.pathname === '/settings') return '设置';
     return 'AI Gateway';
   };
 
@@ -62,27 +64,16 @@ export function AppLayout() {
             onClick={({ key }) => navigate(key)}
             style={{ flex: 1, overflow: 'auto' }}
           />
-          <div
-            style={{
-              padding: '16px 0',
-              display: 'flex',
-              justifyContent: 'center',
-              borderTop: '1px solid rgba(255, 255, 255, 0.1)',
-              flexShrink: 0,
-            }}
-          >
-            <ThemeToggle />
-          </div>
         </div>
       </Layout.Sider>
       <Layout style={{ marginLeft: collapsed ? 80 : 200, transition: 'all 0.2s' }}>
         <Layout.Header
           style={{
             padding: '0 2rem',
-            background: mode === 'dark' ? '#141414' : '#fff',
+            background: effectiveThemeId === 'dark' ? '#141414' : '#fff',
             display: 'flex',
             alignItems: 'center',
-            borderBottom: mode === 'dark' ? '1px solid #303030' : '1px solid #f0f0f0',
+            borderBottom: effectiveThemeId === 'dark' ? '1px solid #303030' : '1px solid #f0f0f0',
           }}
         >
           <h1 style={{ margin: 0, fontSize: '1.25rem' }}>{getPageTitle()}</h1>