feat: 后台 MUI 主题 MD3 化（与前台同源 token/大圆角/surface 边框）+ 深色模式修复

frontend/src/admin/main.jsx

@@ -1,10 +1,12 @@
 import React, { useEffect, useState } from 'react';
 import { createRoot } from 'react-dom/client';
 import { BrowserRouter, Routes, Route, Navigate } from 'react-router-dom';
-import { ThemeProvider, createTheme } from '@mui/material/styles';
+import { ThemeProvider } from '@mui/material/styles';
 import CssBaseline from '@mui/material/CssBaseline';
 import Box from '@mui/material/Box';
 import CircularProgress from '@mui/material/CircularProgress';
+import { createMD3Theme } from './theme.js';
+import './theme.css';
 import { getToken } from '../api/client.js';
 import { me } from '../api/auth.js';
 import { getSettings } from '../api/settings.js';
@@ -32,6 +34,14 @@ function AdminApp() {
   const [status, setStatus] = useState('checking'); // checking | ok
   const [primary, setPrimary] = useState('#6750a4');
 
+  // 深色模式跟随前台偏好：整页跳转后 data-theme 属性不保留，
+  // 以 localStorage 的 theme 为兜底（前台 theme.jsx 持久化同一 key）。
+  const dark = document.documentElement.getAttribute('data-theme') === 'dark'
+    || localStorage.getItem('theme') === 'dark';
+  useEffect(() => {
+    if (dark) document.documentElement.setAttribute('data-theme', 'dark');
+  }, [dark]);
+
   useEffect(() => {
     const token = getToken();
     if (!token) { window.location.href = '/login.html'; return; }
@@ -58,13 +68,7 @@ function AdminApp() {
     );
   }
 
-  const dark = document.documentElement.getAttribute('data-theme') === 'dark';
-  const theme = createTheme({
-    palette: {
-      mode: dark ? 'dark' : 'light',
-      primary: { main: primary },
-    },
-  });
+  const theme = createMD3Theme({ mode: dark ? 'dark' : 'light', primary });
 
   return (
     <ThemeProvider theme={theme}>