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/themeStorage.ts

@@ -0,0 +1,30 @@
+export type ThemeMode = 'light' | 'dark' | 'system';
+
+const THEME_MODE_STORAGE_KEY = 'mumu_theme_mode';
+
+const isThemeMode = (value: string | null): value is ThemeMode => {
+  return value === 'light' || value === 'dark' || value === 'system';
+};
+
+export const getStoredThemeMode = (): ThemeMode => {
+  try {
+    const value = localStorage.getItem(THEME_MODE_STORAGE_KEY);
+    if (isThemeMode(value)) {
+      return value;
+    }
+  } catch (error) {
+    console.warn('读取主题模式失败:', error);
+  }
+
+  return 'system';
+};
+
+export const setStoredThemeMode = (mode: ThemeMode): void => {
+  try {
+    localStorage.setItem(THEME_MODE_STORAGE_KEY, mode);
+  } catch (error) {
+    console.warn('保存主题模式失败:', error);
+  }
+};
+
+export const getThemeModeStorageKey = (): string => THEME_MODE_STORAGE_KEY;