Preserve realtime voice session across immersive toggle

[rogerchappel]

Summary

• Keep a single mounted VoiceAgent while switching between transcript and immersive presentation
• Remove the separate immersive VoiceAgent instance that restarted the realtime relay
• Preserve the existing immersive controls and visual state by changing wrapper styling instead

Verification

• pnpm typecheck
• git diff --check

Risk

Medium. This changes chat page presentation structure for agent voice mode, but keeps behavior scoped to the main chat voice surface and avoids touching realtime protocol code.