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/pages/Settings/index.tsx

@@ -0,0 +1,40 @@
+import { Card, Select, Switch, Space, Typography } from 'antd';
+import { useTheme } from '@/contexts/ThemeContext';
+import { themeOptions, type ThemeId } from '@/themes';
+
+const { Text } = Typography;
+
+export function SettingsPage() {
+  const { themeId, followSystem, setThemeId, setFollowSystem } = useTheme();
+
+  return (
+    <Space direction="vertical" size="large" style={{ width: '100%' }}>
+      <Card title="主题">
+        <Space direction="vertical" size="middle" style={{ width: '100%' }}>
+          <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center' }}>
+            <div>
+              <Text strong>主题</Text>
+            </div>
+            <Select
+              value={themeId}
+              onChange={(value: ThemeId) => setThemeId(value)}
+              style={{ width: 180 }}
+              options={themeOptions.map((opt) => ({
+                value: opt.id,
+                label: opt.label,
+              }))}
+            />
+          </div>
+          <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center' }}>
+            <div>
+              <Text strong>跟随系统</Text>
+              <br />
+              <Text type="secondary">开启后自动跟随系统暗色模式</Text>
+            </div>
+            <Switch checked={followSystem} onChange={setFollowSystem} />
+          </div>
+        </Space>
+      </Card>
+    </Space>
+  );
+}