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/theme/useThemeMode.ts

@@ -0,0 +1,11 @@
+import { useContext } from 'react';
+import { ThemeModeContext } from './themeContext';
+import type { ThemeContextValue } from './themeContext';
+
+export const useThemeMode = (): ThemeContextValue => {
+  const context = useContext(ThemeModeContext);
+  if (!context) {
+    throw new Error('useThemeMode 必须在 ThemeProvider 内使用');
+  }
+  return context;
+};