fix(ui): prefix/suffix 기본값 null 복원 (빈 문자열로 초기화)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/builder.html

@@ -427,8 +427,7 @@ function moveValue(vid,dir){
 function addSetting(nid, key){
   if(!key) return;
   const n=nodeMap[nid]; if(!n||key in n.settings) return;
-  const def = key==='attributes' ? {} : (key==='prefix'||key==='suffix') ? '\n' : '';
-  n.settings[key]=def; refresh();
+  n.settings[key]=key==='attributes'?{}:''; refresh();
 }
 function removeSetting(nid, key){
   const n=nodeMap[nid]; if(!n) return;