refactor: 优化电量监控系统主题与统计逻辑

- 添加深色模式适配，跟随系统主题自动切换
- 替换硬编码样式为CSS变量实现主题统一管理
- 重构后端统计计算逻辑，优化当日用电估算方式
- 更新页面标题与站点图标，调整登录页样式
- 移除冗余的data目录拷贝配置

client/src/main.jsx

@@ -1,17 +1,36 @@
+import { useState, useEffect } from 'react';
 import ReactDOM from 'react-dom/client';
-import { ConfigProvider } from 'antd';
+import { ConfigProvider, theme } from 'antd';
 import App from './App';
 
-ReactDOM.createRoot(document.getElementById('root')).render(
-  <ConfigProvider
-    theme={{
-      token: {
-        colorPrimary: '#4a90e2',
-        borderRadius: 8,
-        fontSize: 14,
-      },
-    }}
-  >
-    <App />
-  </ConfigProvider>
-);
+function Root() {
+  const [isDark, setIsDark] = useState(() =>
+    window.matchMedia('(prefers-color-scheme: dark)').matches
+  );
+
+  useEffect(() => {
+    const mq = window.matchMedia('(prefers-color-scheme: dark)');
+    const handler = (e) => setIsDark(e.matches);
+    mq.addEventListener('change', handler);
+    return () => mq.removeEventListener('change', handler);
+  }, []);
+
+  return (
+    <ConfigProvider
+      theme={{
+        algorithm: isDark ? theme.darkAlgorithm : theme.defaultAlgorithm,
+        token: {
+          colorPrimary: '#4a90e2',
+          borderRadius: 8,
+          fontSize: 14,
+        },
+      }}
+    >
+      <div data-theme={isDark ? 'dark' : 'light'}>
+        <App />
+      </div>
+    </ConfigProvider>
+  );
+}
+
+ReactDOM.createRoot(document.getElementById('root')).render(<Root />);