메신저 사용자 검색 undefined 가드

useCompanyUsers 응답 중 user_name 이 비어있는 레거시 row 가 있을 때
toLowerCase 호출에서 발생하던 Runtime TypeError 방지.

- NewRoomModal: userName/deptName ?? '' 으로 가드 후 includes
- MessageInput: 멘션 자동완성 필터에서 userName ?? '' 가드

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/components/messenger/MessageInput.tsx

@@ -39,7 +39,7 @@ function MessageInput({ roomId, onTypingStart, onTypingStop }: MessageInputProps
   const { data: users } = useCompanyUsers();
 
   const filteredMentionUsers = mentionQuery !== null && users
-    ? users.filter((u) => u.userName.toLowerCase().includes(mentionQuery.toLowerCase())).slice(0, 5)
+    ? users.filter((u) => (u.userName ?? "").toLowerCase().includes(mentionQuery.toLowerCase())).slice(0, 5)
     : [];
 
   // Revoke object URLs when pending files change or component unmounts

frontend/components/messenger/NewRoomModal.tsx

@@ -35,11 +35,12 @@ export function NewRoomModal({ open, onOpenChange, userStatuses }: NewRoomModalP
   const createRoom = useCreateRoom();
   const { selectRoom } = useMessengerContext();
 
-  const filtered = users.filter(
-    (u) =>
-      u.userName.toLowerCase().includes(search.toLowerCase()) ||
-      (u.deptName && u.deptName.toLowerCase().includes(search.toLowerCase()))
-  );
+  const filtered = users.filter((u) => {
+    const q = search.toLowerCase();
+    const name = (u.userName ?? "").toLowerCase();
+    const dept = (u.deptName ?? "").toLowerCase();
+    return name.includes(q) || dept.includes(q);
+  });
 
   const toggleUser = (userId: string) => {
     if (tab === "dm") {