[fix](client): scan QR codes with MLKit's barcode output, not the iOS-only object output

Tapping "Scan a QR code" crashed the app on Android:

    com.facebook.react.common.JavascriptException: Error:
    CameraFactory.createObjectOutput(...): java.lang.Error:
    CameraObjectOutput is not available on Android!

`useObjectOutput` is Apple Vision only — the comment claiming v5 ships a
cross-platform code scanner in core was wrong, and the native factory throws as
soon as the screen mounts. The Barcode Scanner output is the cross-platform one
(MLKit on Android, Vision on iOS); it is attached to the same `<Camera>` via
`outputs`, scans `qr-code` only, and keeps the hand-paste fallback. The scan
callback now takes the barcode values directly instead of the object-output
shape.

src/screens/ScannerScreen.tsx

@@ -14,8 +14,8 @@ import {
   Camera,
   useCameraDevice,
   useCameraPermission,
-  useObjectOutput,
 } from 'react-native-vision-camera';
+import { useBarcodeScannerOutput } from 'react-native-vision-camera-barcode-scanner';
 import { ScanLine, X } from 'lucide-react-native';
 import { AppText, Button, ErrorBanner, Field, Header, TextInputBox } from '../components/ui';
 import { useAlarm } from '../state/AlarmContext';
@@ -158,32 +158,42 @@ export default function ScannerScreen() {
     [busy, joinAlert, navigation, perms],
   );
 
-  const onScanned = useCallback(
-    (objects: readonly { type: string; value?: string }[]) => {
-      for (const obj of objects) {
-        const value = typeof obj.value === 'string' ? obj.value : undefined;
-        if (obj.type !== 'qr' || !value) continue;
-        const parsed = parseQrPayload(value);
-        if (parsed.ok) {
-          if (targetAlertId && parsed.payload.alertId !== targetAlertId) {
-            setError('That code is for a different alert.');
-            continue;
-          }
-          complete(parsed.payload.alertId, parsed.payload.subscribeToken);
-        } else {
-          setError(qrErrorMessage(parsed.error));
+  // Both platforms scan through MLKit here. The core library's
+  // `useObjectOutput` is Apple-Vision only: on Android its native factory
+  // throws `CameraObjectOutput is not available on Android!` the moment the
+  // screen mounts, which crashed the app on "Scan a QR code". The Barcode
+  // Scanner output is cross-platform (MLKit on Android, Vision on iOS).
+  const handleScanValue = useCallback(
+    (value: string) => {
+      const parsed = parseQrPayload(value);
+      if (parsed.ok) {
+        if (targetAlertId && parsed.payload.alertId !== targetAlertId) {
+          setError('That code is for a different alert.');
+          return;
         }
-        return;
+        complete(parsed.payload.alertId, parsed.payload.subscribeToken);
+      } else {
+        setError(qrErrorMessage(parsed.error));
       }
     },
     [complete, targetAlertId],
   );
 
-  // v5 ships the scanner in core: `useObjectOutput` is the code-scanner — there
-  // is no separate vision-camera-code-scanner plugin for 5.x.
-  const objectOutput = useObjectOutput({
-    types: ['qr'],
-    onObjectsScanned: onScanned,
+  const barcodeOutput = useBarcodeScannerOutput({
+    barcodeFormats: ['qr-code'],
+    onBarcodeScanned(barcodes) {
+      for (const barcode of barcodes) {
+        const value = barcode.rawValue;
+        if (typeof value === 'string' && value.length > 0) {
+          handleScanValue(value);
+          return;
+        }
+      }
+    },
+    onError(error) {
+      setError('The camera could not start scanning. You can paste the code by hand.');
+      console.log('barcode scanner error', error.message);
+    },
   });
 
   const submitManual = async () => {
@@ -236,7 +246,7 @@ export default function ScannerScreen() {
             style={StyleSheet.absoluteFill}
             device={device}
             isActive={!joinedName}
-            outputs={[objectOutput]}
+            outputs={[barcodeOutput]}
           />
         ) : (
           <View style={styles.permission}>