fix: replace deprecated BackHandler.removeEventListener with subscription.remove() to prevent crash on unmount

src/screens/AdminScreen.tsx

@@ -125,8 +125,8 @@ export default function AdminScreen({ currentSettings, onSave, onClose }: AdminS
       return true; // Prevent default
     };
 
-    BackHandler.addEventListener('hardwareBackPress', onBackPress);
+    const subscription = BackHandler.addEventListener('hardwareBackPress', onBackPress);
-    return () => BackHandler.removeEventListener('hardwareBackPress', onBackPress);
+    return () => subscription.remove();
   }, [
     countdown, printerIp, password, kioskActive, frameMode, selectedFrameId, availableFrameIds,
     dateOverlay, dateOverlayPosition, eventText, burstCount, burstInterval, welcomeText, 

src/screens/PreviewScreen.tsx

@@ -103,9 +103,9 @@ export default function PreviewScreen({
       onReset();
       return true; // Prevent default behavior (closing the app)
     };
-    BackHandler.addEventListener('hardwareBackPress', onBackPress);
+    const subscription = BackHandler.addEventListener('hardwareBackPress', onBackPress);
     return () => {
-      BackHandler.removeEventListener('hardwareBackPress', onBackPress);
+      subscription.remove();
     };
   }, [onReset]);