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/app.config.ts

@@ -57,6 +57,11 @@ const config: ExpoConfig = {
         color: "#000000",
       },
     ],
+    // LiveKit needs WebRTC native modules. The two plugins below patch the
+    // iOS/Android prebuild so camera/mic permissions and the webrtc pod are
+    // wired up correctly for huddles.
+    "@config-plugins/react-native-webrtc",
+    "@livekit/react-native-expo-plugin",
   ],
   experiments: {
     typedRoutes: false,