Replace language select with toggle menu

Replace the old <select> language picker with a custom toggle button and dropdown menu. Updates include new popup HTML (langToggle button, langMenu with menuitemradio buttons), new CSS for .lang-toggle/.lang-menu/.lang-menu-item (styling, hover/active states, shadow, rounded corners), and JS to manage label sync, menu state, click handlers, outside-click and Escape to close, and applying the selected UI language. Also update references in i18n code (setTitle target and language init flow) and persist language selection. Bump manifest version from 2026.03.24 to 2026.03.25.

README.md

@@ -5,6 +5,7 @@ Chrome 应用商店：<https://chrome.google.com/webstore/detail/memos-bber/cbhj
 一个通过浏览器插件发布 [Memos](https://usememos.com/) 的插件。基于 iSpeak-bber 修改，原作者为 [DreamyTZK](https://www.antmoe.com/)。
 
 ## 更新日志
+- 20260325 优化语言按钮样式
 - 20260323 优化中文显示效果
 - 20260322 适配移动端竖屏窗口
 - 20260310 记忆拖拽窗口大小,移除拖拽窗口动画