tublydownloader/Youtube-Downloader-Extension — reverse-engineered prompt

Build me a browser extension like Tubly Downloader that works on YouTube pages and makes it easy for someone to save videos.

I want a clean download button to appear under normal YouTube videos and on YouTube Shorts. When clicked, it should show simple options for video quality, including 4K when available, and an audio option that saves as a high quality MP3. It should feel lightweight, with no ads or extra software prompts, and should work across Chrome style browsers. If Firefox support is practical, set it up too.

Please include the extension manifest, popup or UI pieces, content script behavior for detecting YouTube videos and Shorts, icons, and a basic build setup. Make the interface simple and polished, with clear status messages like loading, downloading, error, and finished. Also include installation instructions for loading it as an unpacked extension.

Look up current browser extension docs online if you need to.