feat(mobile): add huddles with LiveKit

Mirrors the desktop huddle window: tap the headphones button on a stream
to fetch a LiveKit token from Orion and join a video room. Active huddles
show a red badge on the stream card (participant count) and a red Join
pill in the stream top bar — both driven by huddle_active_participants
synced from the LiveKit webhook.

The HuddleScreen lives at its own route (not a modal/window): video tile
grid with placeholders for camera-off, mic/camera toggles, and a leave
button. Adds the LiveKit RN SDK plus the WebRTC + LiveKit Expo config
plugins for prebuild.

https://claude.ai/code/session_01Po5tAD17MXhnqGQ9hKh6PC

js/mobile/index.ts

@@ -1,5 +1,11 @@
 import "./global.css";
+import { registerGlobals } from "@livekit/react-native";
 import { registerRootComponent } from "expo";
 import App from "./src/App";
 
+// LiveKit pipes WebRTC globals (RTCPeerConnection, mediaDevices, ...) onto the
+// JS global. Must run before any LiveKit client is constructed, so do it at
+// app entry rather than inside the huddle screen.
+registerGlobals();
+
 registerRootComponent(App);