fix LLM explainer for sentence: section + example sentences

docs/.vitepress/theme/components/VocabQuiz.vue

@@ -187,9 +187,11 @@ const example = computed(() => {
   return ex[exIndex.value] || null;
 });
 
-const sent_ko = computed(
-  () => (example.value?.ko || fmKey !== "vocabulary" ? v.value?.ko : "") || "",
-);
+const sent_ko = computed(() => {
+  if (example.value) return example.value.ko || "";
+  if (fmKey === "vocabulary") return "";
+  return v.value?.ko || "";
+});
 
 const MAX_VOC = 5;