Handle unsupported codecs

Some MPEG-4 profiles are not supported on every platform, such as h264 (High 10) on Firefox for Windows 11.
We could prompt the user to install the required codec or use a different browser.
Alternatively, we could transcode the media file with ffmpeg but this likely leads to a degraded user-experience since transcoding is highly cpu-intensive, especially on lower-end devices.

The issue names no files, tests, or entry points. Start by locating the browser playback and codec-detection flow, then clarify whether the intended done state is a user prompt, browser fallback, or ffmpeg-based transcoding.

Written by the indexing model from the issue text.