feat(学生视图): 添加imgExporting到全局导出功能

将imgExporting方法添加到PPTistStudent全局对象中，以便在外部调用图片导出功能

src/views/Student/index2.vue

@@ -1255,7 +1255,7 @@ const clearAllSyncStates = () => {
 }
 
 // 获取导入导出功能
-const { readJSON, exportJSON2, getFile, getFile2 } = useImport()
+const { readJSON, exportJSON2, getFile, getFile2, imgExporting } = useImport()
 
 // 根据iframe的URL查找对应的幻灯片索引
 const findSlideIndexByIframeUrl = (iframeUrl: string): number => {
@@ -3379,6 +3379,7 @@ onMounted(() => {
   ; (window as any).PPTistStudent = {
     importJSON,
     exportJSON,
+    imgExporting,
     slides: slidesStore.slides,
     currentSlide: computed(() => slidesStore.currentSlide),
     slideIndex: computed(() => slidesStore.slideIndex),