fix: use preview-mode signed URLs and graceful media preview fallback

frontend/app.js

@@ -1937,7 +1937,10 @@ handleDragLeave(e) {
       if (this.storageType === 'oss' && this.user?.oss_config_source !== 'none') {
         try {
           const { data } = await axios.get(`${this.apiBase}/api/files/download-url`, {
-            params: { path: filePath }
+            params: {
+              path: filePath,
+              mode: 'preview'
+            }
           });
           if (data.success) {
             return data.downloadUrl;
@@ -1995,6 +1998,17 @@ handleDragLeave(e) {
       }
     },
 
+    handleMediaPreviewError(type = 'file') {
+      const typeTextMap = {
+        image: '图片',
+        video: '视频',
+        audio: '音频'
+      };
+      const typeText = typeTextMap[type] || '文件';
+      this.showToast('error', '预览失败', `${typeText}预览失败，请尝试下载后查看`);
+      this.closeMediaViewer();
+    },
+
     // 打开视频播放器
     async openVideoPlayer(file) {
       const url = await this.getMediaUrl(file);