feat: 侧边栏默认亮色主题，暗黑主题下跟随切换

frontend/src/components/AppLayout/index.tsx

@@ -15,6 +15,7 @@ export function AppLayout() {
   const location = useLocation();
   const navigate = useNavigate();
   const { effectiveThemeId } = useTheme();
+  const isDark = effectiveThemeId === 'dark';
   const [collapsed, setCollapsed] = useState(false);
 
   const getPageTitle = () => {
@@ -31,6 +32,7 @@ export function AppLayout() {
         collapsed={collapsed}
         onCollapse={setCollapsed}
         breakpoint="lg"
+        theme={isDark ? 'dark' : 'light'}
         style={{
           overflow: 'hidden',
           height: '100vh',
@@ -47,7 +49,7 @@ export function AppLayout() {
               display: 'flex',
               alignItems: 'center',
               justifyContent: 'center',
-              color: '#fff',
+              color: isDark ? '#fff' : 'rgba(0, 0, 0, 0.88)',
               fontSize: collapsed ? '1rem' : '1.25rem',
               fontWeight: 600,
               transition: 'all 0.2s',
@@ -57,7 +59,7 @@ export function AppLayout() {
             {collapsed ? 'AI' : 'AI Gateway'}
           </div>
           <Menu
-            theme="dark"
+            theme={isDark ? 'dark' : 'light'}
             mode="inline"
             selectedKeys={[location.pathname]}
             items={menuItems}