닉네임 제약 조건 추가

@@ -77,6 +77,10 @@ export class Connection {
   private handleLogin(
     message: ServerInboundMessage<"login">
   ): ServerResponse<"login"> {
+    if (message.nickname.length > 12 || message.nickname.trim().length === 0) {
+      return { ok: false };
+    }
+
     this.user = new User(message.nickname, this);
     // console.log(`User ${message.username} has logged in!`);