[Refactor/Chore] Replace lamejs MP3 encoding with Mediabunny

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## Description

Replace the web voice input MP3 conversion dependency from `lamejs` to Mediabunny and `@mediabunny/mp3-encoder`. The voice input flow should continue to convert recorder PCM data into an `audio/mp3` Blob before uploading it for speech-to-text.

## Motivation

`lamejs` requires local type declarations and deep imports to work around runtime/global issues. Mediabunny provides a maintained MP3 encoder extension with a public API for encoding raw audio samples, which should make the voice input conversion path easier to maintain.

## Additional Context

This affects the frontend voice input utility and its tests only. The intended behavior is functionally equivalent: record PCM audio, encode MP3 at 128 kbps, and upload the resulting MP3 file.