add voice input and manual read-aloud

[zatevakhin]

Summary

Add voice input to the composer and a read-aloud button under completed assistant turns.

Scope

• Transcribe voice input into the composer without automatically submitting it.
• Add a play/stop control under assistant responses.
• Show recording, transcription, playback, and error states.
• Gate the controls on available speech capabilities.

Automatic voice output from the agent loop is out of scope.