🐛(front) fix CalDavService return type

Fix return type for respondToInvitation method to properly
wrap response in data property as expected by CalDavResponse.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/frontend/apps/calendars/src/features/calendar/services/dav/CalDavService.ts

@@ -929,11 +929,14 @@ export class CalDavService {
     }
 
     return {
+      success: true,
+      data: {
         success: true,
         responses: [{
           recipient: event.organizer?.email || '',
           status: 'delivered' as const,
         }],
+      },
     }
   }