fix: 上一题/下一题不再调 AI API，改为保存后关闭并推进索引

src/components/FloatingToolbar.vue

@@ -118,12 +118,13 @@ async function startGrading() {
   const questions = rubricData?.questions?.length
     ? JSON.parse(JSON.stringify(rubricData.questions))
     : [{ questionTitle: '', maxScore: 100, referenceAnswer: '', rubric: '' }]
+  const nextIdx = Math.min(await window.electronAPI?.rubric.getNextIndex() ?? 0, questions.length - 1)
   gradingStore.setQuestions(questions)
-  gradingStore.setCurrentQuestionIndex(0)
+  gradingStore.setCurrentQuestionIndex(nextIdx)
 
   appStore.isGrading = true
   try {
-    const result = await gradeQuestion(0)
+    const result = await gradeQuestion(nextIdx)
     if (!result) return
 
     gradingStore.setResult(result)
@@ -133,7 +134,7 @@ async function startGrading() {
         result,
         image: gradingStore.currentImage,
         questions,
-        currentQuestionIndex: 0
+        currentQuestionIndex: nextIdx
       })
       await window.electronAPI.dialog.open('result')
     }