MP4 to AAC

Pull the audio track out of an MP4 and save it as a raw .aac file. The video stream is discarded and, on the happy path, the audio isn't re-encoded at all — ffmpeg's `-c copy` simply repackages the existing AAC stream from the MP4 container into a bare AAC file. That makes this nearly instant, even for long clips, and means there's zero generation loss: the bytes that go in are the bytes that come out. Almost every MP4 uses AAC audio, so this works for the vast majority of files. If a particular MP4 has an unusual audio codec (or a stream that can't be copied directly), the conversion will surface a clear error and you can use mp4-to-mp3 instead. The conversion runs entirely in your browser via ffmpeg.wasm — nothing is uploaded.