feat(StudyDetailPage): 添加PDF预览功能

引入@sunsetglow/vue-pdf-viewer库，实现在学习详情页面中直接预览PDF文件的功能，提升用户体验。移除原有的PDF文件下载链接，改为点击“查看文件”按钮即可在页面内直接预览PDF内容。

@@ -19,6 +19,7 @@
     "@fortawesome/free-solid-svg-icons": "^6.5.1",
     "@fortawesome/vue-fontawesome": "^3.0.5",
     "@heroicons/vue": "^2.2.0",
+    "@sunsetglow/vue-pdf-viewer": "^0.3.0",
     "@vant/touch-emulator": "^1.4.0",
     "@vant/use": "^1.6.0",
     "@videojs-player/vue": "^1.0.0",