Download YouTube videos as MP4 at highest quality. Use when user sends a YouTube URL and wants to download/save it. Triggers on YouTube links (youtube.com, youtu.be) with download intent. Stores videos as assets with labels and registers them for the dashboard.

将YouTube视频以最高画质下载为MP4格式。当用户发送YouTube链接并希望下载保存时使用。识别YouTube链接（youtube.com、youtu.be）并判断有下载意图。将视频作为资源存储并添加标签，同时在仪表盘中注册。