fix: 어드민 한 번 보내진 메일은 수정 불가하게 disable 처리 (#9)

Author: choyeon2e

apps/recruit/src/app/admin/(with-sidebar)/recruitment/_components/manage-mail/ManageMail.tsx

@@ -60,7 +60,7 @@ export const ManageMail = ({
   const canAccess = isGenerationExist || alwaysAble;
   const hasPermission = isRecruiting || alwaysAble;
 
-  const finalCanEdit = canAccess;
+  const finalCanEdit = canAccess && !isSent;
   const finalCanSend =
     canAccess && hasPermission && !isSent && waitingCount > 0;