ffmpegwasm/ffmpeg.wasm ? reverse-engineered prompt

Build me a browser app that lets people upload a video or audio file and convert it right in the browser, without sending the file to a server.

I want a simple, friendly interface where someone can pick a file, choose an output format or preset, start the conversion, and see progress while it runs. It should also be able to trim or transcode media and let me download the finished file when it is done. If possible, include a small demo page with a clean layout, basic instructions, and a few common examples like converting video to MP4 or extracting audio to MP3.

Use the ffmpeg wasm approach so everything happens locally in the browser. Make it feel polished and easy to use, with clear error messages if a file is too big or the browser does not support something. If you need to check the current docs, go ahead and look them up online.