fix: 参考答案图片存完整 data URL (含 mime)，修复无图时提示词顺序一致化

src/components/ResultDrawer.vue

@@ -28,7 +28,7 @@
         <div class="result-section" v-if="currentQuestion.referenceAnswer">
           <div class="section-title">参考答案</div>
           <div class="reference-text">{{ currentQuestion.referenceAnswer }}</div>
-          <img v-if="currentQuestion.referenceAnswerImage" class="ref-image" :src="'data:image/png;base64,' + currentQuestion.referenceAnswerImage" />
+          <img v-if="currentQuestion.referenceAnswerImage" class="ref-image" :src="currentQuestion.referenceAnswerImage" />
         </div>
 
         <div class="result-score">

src/components/RubricEditor.vue

@@ -142,7 +142,7 @@ async function uploadRefImage(idx: number) {
   if (!window.electronAPI) return
   const data = await window.electronAPI.dialog.openImage()
   if (data) {
-    questions.value[idx].referenceAnswerImage = data.base64
+    questions.value[idx].referenceAnswerImage = `data:${data.mime};base64,${data.base64}`
   }
 }