feat(file-preview): 增加PDF文件预览功能并优化预览逻辑

- 引入统一的文件预览工具函数和类型定义
- 添加PDF文件类型的识别和预览支持
- 使用iframe实现PDF文件在线预览
- 重构文件预览逻辑，统一处理不同文件类型的预览
- 优化文本内容预览的长度截取机制
- 更新预览按钮加载状态显示
- 统一预览窗口的最大高度配置
- 修改API调用路径为专门的预览接口

frontend/src/pages/KnowledgeManagement/Detail/KnowledgeSetDetail.tsx

@@ -41,7 +41,12 @@ import CreateKnowledgeSet from "../components/CreateKnowledgeSet";
 import KnowledgeItemEditor from "../components/KnowledgeItemEditor";
 import ImportKnowledgeItemsDialog from "../components/ImportKnowledgeItemsDialog";
 import { formatDate } from "@/utils/unit";
-import { PREVIEW_TEXT_MAX_LENGTH, resolvePreviewFileType, truncatePreviewText } from "@/utils/filePreview";
+import {
+  PREVIEW_TEXT_MAX_LENGTH,
+  resolvePreviewFileType,
+  truncatePreviewText,
+  type PreviewFileType,
+} from "@/utils/filePreview";
 
 const PREVIEW_MAX_HEIGHT = 500;
 const PREVIEW_MODAL_WIDTH = {
@@ -67,7 +72,7 @@ const KnowledgeSetDetail = () => {
   const [previewVisible, setPreviewVisible] = useState(false);
   const [previewContent, setPreviewContent] = useState("");
   const [previewFileName, setPreviewFileName] = useState("");
-  const [previewFileType, setPreviewFileType] = useState<"text" | "image" | "video" | "audio">("text");
+  const [previewFileType, setPreviewFileType] = useState<PreviewFileType>("text");
   const [previewMediaUrl, setPreviewMediaUrl] = useState("");
   const [previewLoadingItemId, setPreviewLoadingItemId] = useState<string | null>(null);
 
@@ -560,6 +565,13 @@ const KnowledgeSetDetail = () => {
             />
           </div>
         )}
+        {previewFileType === "pdf" && (
+          <iframe
+            src={previewMediaUrl}
+            title={previewFileName || "PDF 预览"}
+            style={{ width: "100%", height: `${PREVIEW_MAX_HEIGHT}px`, border: "none" }}
+          />
+        )}
         {previewFileType === "video" && (
           <div style={{ textAlign: "center" }}>
             <video