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/src/navigation/RootNavigator.tsx

@@ -6,6 +6,7 @@ import { NetworkListScreen } from "@/features/networks/NetworkListScreen";
 import { StreamListScreen } from "@/features/streams/StreamListScreen";
 import { NewStreamScreen } from "@/features/streams/NewStreamScreen";
 import { StreamViewScreen } from "@/features/stream-view/StreamViewScreen";
+import { HuddleScreen } from "@/features/huddle/HuddleScreen";
 import { SettingsScreen } from "@/features/settings/SettingsScreen";
 import { AccountScreen } from "@/features/settings/AccountScreen";
 import type { RootStackParamList } from "./types";
@@ -43,6 +44,11 @@ export function RootNavigator() {
         component={StreamViewScreen}
         options={{ animation: "fade", gestureEnabled: false }}
       />
+      <Stack.Screen
+        name="Huddle"
+        component={HuddleScreen}
+        options={{ animation: "slide_from_bottom", gestureEnabled: false }}
+      />
       <Stack.Screen
         name="NewStream"
         component={NewStreamScreen}