wip(mobile): in-progress mobile + voice polish

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.

packages/web/src/components/voice/VoiceUser.tsx

@@ -141,6 +141,7 @@ export function VoiceUser({ tile, large }: VoiceUserProps) {
 
   return (
     <div
+      data-context-menu
       className={`relative bg-surface-base rounded-xl overflow-hidden flex items-center justify-center group transition-all duration-200 ${
         isSpeaking
           ? 'ring-[3px] ring-status-online shadow-[0_0_12px_rgba(134,239,172,0.25)]'