feat(mobile): mount CallProvider + global IncomingCallModal

apps/mobile/app/_layout.tsx

@@ -5,13 +5,11 @@ import { GestureHandlerRootView } from 'react-native-gesture-handler';
 import { SafeAreaProvider } from 'react-native-safe-area-context';
 
 import { ErrorBoundary } from '../components/ErrorBoundary';
+import { IncomingCallModal } from '../components/IncomingCallModal';
 import { AuthProvider } from '../lib/authContext';
+import { CallProvider } from '../lib/callContext';
 import { createLibsodiumBackend } from '../lib/cryptoBackend';
 
-// Register the crypto backend exactly once, before any code that calls
-// crypto.getCryptoBackend(). Doing it at module load (not inside the
-// component) avoids a race where a child component renders before the
-// effect fires.
 crypto.setCryptoBackend(createLibsodiumBackend());
 
 export default function RootLayout() {
@@ -20,12 +18,15 @@ export default function RootLayout() {
       <SafeAreaProvider>
         <ErrorBoundary>
           <AuthProvider>
-            <StatusBar style="auto" />
-            <Stack screenOptions={{ headerShown: false }}>
-              <Stack.Screen name="index" />
-              <Stack.Screen name="(app)" />
-              <Stack.Screen name="auth/callback" />
-            </Stack>
+            <CallProvider>
+              <StatusBar style="auto" />
+              <Stack screenOptions={{ headerShown: false }}>
+                <Stack.Screen name="index" />
+                <Stack.Screen name="(app)" />
+                <Stack.Screen name="auth/callback" />
+              </Stack>
+              <IncomingCallModal />
+            </CallProvider>
           </AuthProvider>
         </ErrorBoundary>
       </SafeAreaProvider>