fix: 学校端导出接口空数据时显示具体错误消息

- 修改 DashboardView.vue 中三个导出方法的错误处理逻辑
- 导出授课记录/教师绩效/学生统计时，空数据场景下显示后端返回的具体提示
- 错误消息从"导出失败"改为使用后端返回的详细消息内容

影响范围：
- reading-platform-frontend/src/views/school/DashboardView.vue
  - handleExportLessons
  - handleExportTeacherStats
  - handleExportStudentStats

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

reading-platform-frontend/src/views/school/DashboardView.vue

@@ -698,7 +698,10 @@ const handleExportLessons = async () => {
     await exportLessons();
     message.success({ content: '导出成功', key: 'export' });
   } catch (error) {
-    message.error({ content: '导出失败', key: 'export' });
+    message.error({
+      content: error instanceof Error ? error.message : '导出失败',
+      key: 'export'
+    });
   }
 };
 
@@ -708,7 +711,10 @@ const handleExportTeacherStats = async () => {
     await exportTeacherStats();
     message.success({ content: '导出成功', key: 'export' });
   } catch (error) {
-    message.error({ content: '导出失败', key: 'export' });
+    message.error({
+      content: error instanceof Error ? error.message : '导出失败',
+      key: 'export'
+    });
   }
 };
 
@@ -718,7 +724,10 @@ const handleExportStudentStats = async () => {
     await exportStudentStats();
     message.success({ content: '导出成功', key: 'export' });
   } catch (error) {
-    message.error({ content: '导出失败', key: 'export' });
+    message.error({
+      content: error instanceof Error ? error.message : '导出失败',
+      key: 'export'
+    });
   }
 };