fix(ThemeModeProvider): 修复主题快照同步与首屏闪烁问题

抽取 themeSnapshot 工具模块，异步恢复与跨窗同步时写回 snapshot；HTML 内联脚本与 useLayoutEffect 消除 dark 类首帧闪烁；避免 storage 恢复覆盖用户操作及 setState 嵌套更新。

Co-authored-by: Cursor <cursoragent@cursor.com>

src/entrypoints/popup/main.tsx

@@ -1,8 +1,11 @@
 import ReactDOM from 'react-dom/client';
+import { applyThemeFromSnapshot } from '@/utils/themeSnapshot';
 import AppRoot from '@/providers/AppRoot';
 import '@/index.css';
 import App from './App.tsx';
 
+applyThemeFromSnapshot();
+
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <AppRoot>
     <App />