feat(mobile): register /call full-screen modal route

apps/mobile/app/(app)/_layout.tsx

@@ -2,13 +2,18 @@ import { Redirect, Stack } from 'expo-router';
 
 import { useAuth } from '../../lib/authContext';
 
-// Guard for the authenticated route group. While the AuthProvider is
-// hydrating from AsyncStorage we render nothing (a brief blank frame)
-// rather than flashing the login screen and then snapping into the chat
-// list — that's a worse UX than a 200ms blank.
 export default function AppLayout() {
   const { session, loading } = useAuth();
   if (loading) return null;
   if (!session) return <Redirect href="/" />;
-  return <Stack screenOptions={{ headerShown: true }} />;
+  return (
+    <Stack screenOptions={{ headerShown: true }}>
+      <Stack.Screen name="chats" />
+      <Stack.Screen name="conversations/[id]" />
+      <Stack.Screen
+        name="call"
+        options={{ headerShown: false, presentation: 'fullScreenModal' }}
+      />
+    </Stack>
+  );
 }