Fix Firefox WebUI playback by falling back when audio/mpeg MSE is unsupported

[JiminyScript]

Summary

The integrated WebUI currently assumes MP3 can always be streamed through MediaSource:

• response_format: 'mp3'
• stream: true
• addSourceBuffer('audio/mpeg')

This breaks in Firefox, where MediaSource.isTypeSupported('audio/mpeg') returns false and playback fails with:

• MediaSource.addSourceBuffer: Type not supported in MediaSource
• Cannot play media. No decoders for requested formats: audio/mpeg

What this change does

• Detects support for MP3 MediaSource playback with MediaSource.isTypeSupported('audio/mpeg')
• Uses streaming only when supported
• Falls back to normal blob-backed <audio> playback otherwise
• Adds object URL cleanup
• Adds a few null guards to avoid follow-on errors after playback setup failure

Result

• Chromium browsers keep streaming playback
• Firefox works instead of failing
• No browser sniffing required