Snapshot of in-progress work parked here so group-DM-polish can land cleanly on main. Touches MainContent + mobile-ui.md which overlap with group-DM-polish; rebase onto post-merge main and resolve conflicts on those two files manually. Files: MainContent, MessageInput, MobileVoiceFullScreen (+test), StreamTile, VoiceUser, useLiveKit, useVisualViewportInset, AudioManager, voiceStore, voice utils, mobile-ui.md, voice.md, mobile-parity handoff doc.