MiMo ASR (mimo-v2.5-asr) only accepts audio/wav, audio/mp3, and
audio/mpeg formats. Web browsers record in WebM/Opus by default,
causing the API to reject the payload with a transcription error.
This change adds a frontend WebM→WAV converter using the Web Audio API
(DecodeAudioData + PCM encoding) that activates only when the
configured transcription provider is 'xiaomi_mimo'. Other providers
are unaffected — they continue to receive the original browser format.
Tested and confirmed working on WebUI.