feat(web): voice mode overlay talks to the bench's worker with browser speech (CL-9568)

[TheGreatAxios]

Adds a voice overlay over the bench thread, opened from a new composer voice button.

• SpeechRecognition (continuous, interim) sends each final utterance through the composer's send path as an ordinary message.
• New worker replies that arrive while voice is open are spoken with speechSynthesis; recognition pauses while speaking.
• Mic level (AnalyserNode) drives the orb glow; captions toggle, mute (Space), end (Esc).
• Unsupported browsers and denied mic show an inline message.

Resolves CL-9568

[linear-code]

CL-9568