feat: add theme toggle functionality and update localization files

- Implemented a theme toggle feature allowing users to switch between light and dark modes.
- Added a new ThemeToggle component for the theme switching button.
- Updated theme CSS to support dark mode variables and system preference detection.
- Enhanced localization files to include translations for the "toggleTheme" key across multiple languages.

README.md

@@ -172,6 +172,16 @@ docker compose up -d
 
 ---
 
+## Dark Mode
+
+ConvertX-CN 支援亮色與暗色主題。
+
+- 可手動切換主題（點擊導覽列的太陽/月亮圖示）
+- 偏好設定自動儲存於瀏覽器
+- 預設跟隨系統色彩偏好
+
+---
+
 ## MinerU
 
 ConvertX 內建文件轉換引擎。