Convert HEIC gallery photos to JPEG on iOS

Use the compatible asset representation so the picker transcodes HEIC
to JPEG before upload, a format the backend resizes and browsers display.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/(protected)/activity/add.tsx

@@ -117,6 +117,8 @@ export default function ActivityFormScreen() {
                 allowsMultipleSelection: true,
                 selectionLimit: limit,
                 quality: 0.8,
+                // iOS: converte HEIC in JPEG, formato gestito dal backend e visualizzabile sul web
+                preferredAssetRepresentationMode: ImagePicker.UIImagePickerPreferredAssetRepresentationMode.Compatible,
             });
 
             if (!result.canceled && result.assets) {