fix(backup): improve error messages to distinguish EBUSY from format errors

Closes #12160

**Changes**:
- Enhanced error handling in BackupService.ts (restore, restoreFromWebdav, restoreFromLocal)
- Added specific error messages for EBUSY (file locked) and ENOENT (file not found)
- Added i18n support for new error messages in en-us.json and zh-cn.json

**Impact**:
Users will now see accurate error messages:
- EBUSY errors → "Cannot restore: Some files are in use. Please close all knowledge bases or restart the app and try again."
- ENOENT errors → "Backup file not found or is corrupted"
- JSON errors → "Backup file format error"
- Other errors → Detailed error message for debugging

**Technical Details**:
- Distinguishes error codes (EBUSY, ENOENT) and message content
- Provides clear user guidance instead of misleading "file format error"
- Step 1 of phased fix plan for #12160

src/renderer/src/i18n/locales/en-us.json

@@ -1309,7 +1309,9 @@
     "availableProviders": "Available Providers",
     "availableTools": "Available Tools",
     "backup": {
-      "file_format": "Backup file format error"
+      "file_format": "Backup file format error",
+      "file_locked": "Cannot restore: Some files are in use. Please close all knowledge bases or restart the app and try again.",
+      "file_not_found": "Backup file not found or is corrupted"
     },
     "base64DataTruncated": "Base64 image data truncated, size",
     "boundary": {

src/renderer/src/i18n/locales/zh-cn.json

@@ -1309,7 +1309,9 @@
     "availableProviders": "可用提供商",
     "availableTools": "可用工具",
     "backup": {
-      "file_format": "备份文件格式错误"
+      "file_format": "备份文件格式错误",
+      "file_locked": "无法恢复：某些文件正在使用中。请关闭所有知识库或重启应用后重试。",
+      "file_not_found": "备份文件未找到或已损坏"
     },
     "base64DataTruncated": "Base64 图片数据已截断，大小",
     "boundary": {

src/renderer/src/services/BackupService.ts

@@ -100,9 +100,23 @@ export async function restore() {
         source: 'backup',
         channel: 'system'
       })
-    } catch (error) {
+    } catch (error: any) {
       logger.error('restore: Error restoring backup file:', error as Error)
-      window.toast.error(i18n.t('error.backup.file_format'))
+
+      // Distinguish different error types for better user experience
+      const errorCode = error.code || ''
+      const errorMessage = error.message || ''
+
+      if (errorCode === 'EBUSY' || errorMessage.includes('EBUSY') || errorMessage.includes('locked')) {
+        window.toast.error(i18n.t('error.backup.file_locked'))
+      } else if (errorCode === 'ENOENT' || errorMessage.includes('no such file')) {
+        window.toast.error(i18n.t('error.backup.file_not_found'))
+      } else if (errorMessage.includes('JSON')) {
+        window.toast.error(i18n.t('error.backup.file_format'))
+      } else {
+        // Show detailed error message for debugging
+        window.toast.error(i18n.t('error.backup.file_format') + ': ' + errorMessage)
+      }
     }
   }
 }
@@ -314,9 +328,23 @@ export async function restoreFromWebdav(fileName?: string) {
 
   try {
     await handleData(JSON.parse(data))
-  } catch (error) {
+  } catch (error: any) {
     logger.error('[Backup] Error downloading file from WebDAV:', error as Error)
-    window.toast.error(i18n.t('error.backup.file_format'))
+
+    // Distinguish different error types for better user experience
+    const errorCode = error.code || ''
+    const errorMessage = error.message || ''
+
+    if (errorCode === 'EBUSY' || errorMessage.includes('EBUSY') || errorMessage.includes('locked')) {
+      window.toast.error(i18n.t('error.backup.file_locked'))
+    } else if (errorCode === 'ENOENT' || errorMessage.includes('no such file')) {
+      window.toast.error(i18n.t('error.backup.file_not_found'))
+    } else if (errorMessage.includes('JSON')) {
+      window.toast.error(i18n.t('error.backup.file_format'))
+    } else {
+      // Show detailed error message for debugging
+      window.toast.error(i18n.t('error.backup.file_format') + ': ' + errorMessage)
+    }
   }
 }
 
@@ -1071,9 +1099,24 @@ export async function restoreFromLocal(fileName: string) {
     await handleData(data)
 
     return true
-  } catch (error) {
+  } catch (error: any) {
     logger.error('[LocalBackup] Restore failed:', error as Error)
-    window.toast.error(i18n.t('error.backup.file_format'))
+
+    // Distinguish different error types for better user experience
+    const errorCode = error.code || ''
+    const errorMessage = error.message || ''
+
+    if (errorCode === 'EBUSY' || errorMessage.includes('EBUSY') || errorMessage.includes('locked')) {
+      window.toast.error(i18n.t('error.backup.file_locked'))
+    } else if (errorCode === 'ENOENT' || errorMessage.includes('no such file')) {
+      window.toast.error(i18n.t('error.backup.file_not_found'))
+    } else if (errorMessage.includes('JSON')) {
+      window.toast.error(i18n.t('error.backup.file_format'))
+    } else {
+      // Show detailed error message for debugging
+      window.toast.error(i18n.t('error.backup.file_format') + ': ' + errorMessage)
+    }
+
     throw error
   }
 }