fix: precheck local downloads to avoid JSON file download on quota errors

backend/server.js

@@ -4813,6 +4813,67 @@ app.post('/api/upload', authMiddleware, upload.single('file'), async (req, res)
   }
 });
 
+// 下载预检（避免前端直接下载到 JSON 错误响应）
+app.get('/api/files/download-check', authMiddleware, async (req, res) => {
+  const filePath = req.query.path;
+  let storage;
+
+  if (!filePath) {
+    return res.status(400).json({
+      success: false,
+      message: '缺少文件路径参数'
+    });
+  }
+
+  const normalizedPath = path.posix.normalize(filePath);
+  if (normalizedPath.includes('..') || filePath.includes('\x00')) {
+    return res.status(400).json({
+      success: false,
+      message: '文件路径非法'
+    });
+  }
+
+  try {
+    const policyState = enforceDownloadTrafficPolicy(req.user.id, 'download_check');
+    const latestUser = policyState?.user || UserDB.findById(req.user.id);
+    if (!latestUser) {
+      return res.status(401).json({
+        success: false,
+        message: '用户不存在'
+      });
+    }
+    const trafficState = getDownloadTrafficState(latestUser);
+
+    const { StorageInterface } = require('./storage');
+    const storageInterface = new StorageInterface(req.user);
+    storage = await storageInterface.connect();
+
+    const fileStats = await storage.stat(normalizedPath);
+    const fileSize = Math.max(0, Number(fileStats?.size) || 0);
+    const fileName = normalizedPath.split('/').pop() || 'download.bin';
+
+    if (!trafficState.isUnlimited && fileSize > trafficState.remaining) {
+      return res.status(403).json({
+        success: false,
+        message: `下载流量不足：文件 ${formatFileSize(fileSize)}，剩余 ${formatFileSize(trafficState.remaining)}`
+      });
+    }
+
+    res.json({
+      success: true,
+      fileName,
+      fileSize
+    });
+  } catch (error) {
+    console.error('下载预检失败:', error);
+    res.status(500).json({
+      success: false,
+      message: getSafeErrorMessage(error, '下载检查失败，请稍后重试', '下载预检')
+    });
+  } finally {
+    if (storage) await storage.end();
+  }
+});
 
 // 下载文件
 app.get('/api/files/download', authMiddleware, async (req, res) => {