feature: 新增全局主题系统（浅色/深色/跟随系统）、主题持久化与主题切换组件

frontend/src/main.tsx

@@ -1,42 +1,14 @@
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
-import { ConfigProvider } from 'antd'
-import zhCN from 'antd/locale/zh_CN'
 import 'antd/dist/reset.css'
 import './index.css'
 import App from './App.tsx'
+import { ThemeProvider } from './theme/ThemeProvider'
 
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
-    <ConfigProvider
-      locale={zhCN}
-      theme={{
-        token: {
-          colorPrimary: '#4D8088', // 天青
-          colorBgBase: '#F8F6F1', // 米汤色
-          colorTextBase: '#2B2B2B', // 墨色
-          borderRadius: 6,
-          wireframe: false,
-          fontFamily: "-apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, 'Noto Sans', sans-serif",
-        },
-        components: {
-          Layout: {
-            bodyBg: '#F8F6F1',
-            headerBg: '#FFFFFF',
-            siderBg: '#FFFFFF',
-          },
-          Card: {
-            colorBgContainer: '#FFFFFF',
-            boxShadowTertiary: '0 4px 12px rgba(0, 0, 0, 0.05)', // 更柔和的阴影
-          },
-          Button: {
-            borderRadius: 6,
-            controlHeight: 36,
-          }
-        }
-      }}
-    >
+    <ThemeProvider>
       <App />
-    </ConfigProvider>
+    </ThemeProvider>
   </StrictMode>,
 )