feat: Excel 导入预检与动态问答，AI 聊天/文件解析体验修复

- imports: 新增 preflight_import 预检报告（判定/分阶段统计/阻断归因/问题/下一步/错误示例），导入任务 settings 落库（映射/校区/更新/重复/未匹配策略），预览应用策略，向导提交写操作日志
- ai-chat: 新增 excel_analyze（ExcelJS）工具，移除附件/上下文截断，start_import_wizard 支持确认参数，ui.import_preflight SSE，预览确认写操作日志
- admin: ImportPreflightCard 渲染与持久化，聊天抽屉布局/侧边栏修复，考勤页 CSS 引入，费用/学生页接口 schema 校验修复

apps/server/src/imports/imports.workbook.ts

@@ -1,4 +1,6 @@
+import { BadRequestException } from '@nestjs/common';
 import * as ExcelJS from 'exceljs';
+import { Readable } from 'node:stream';
 import { cellValue, textValue } from './imports.helpers';
 import type { CellValue } from './imports.types';
 
@@ -37,3 +39,50 @@ export function extractSheets(workbook: ExcelJS.Workbook): ImportSheetData[] {
   }
   return sheets;
 }
+
+export type WorkbookKind = 'csv' | 'xlsx';
+
+export function detectWorkbookKind(originalName: string, mimeType: string): WorkbookKind | null {
+  const isCsv =
+    /\.csv$/i.test(originalName) ||
+    /csv/i.test(mimeType) ||
+    /text\/(csv|plain)/i.test(mimeType);
+  const isXlsx =
+    /\.xlsx$/i.test(originalName) ||
+    /spreadsheetml/i.test(mimeType) ||
+    /excel/i.test(mimeType);
+  if (isCsv) return 'csv';
+  if (isXlsx) return 'xlsx';
+  return null;
+}
+
+/** 校验文件类型并解析为工作表数据；解析失败抛出可读错误。 */
+export async function parseSheets(
+  buffer: Buffer,
+  originalName: string,
+  mimeType: string,
+): Promise<ImportSheetData[]> {
+  const kind = detectWorkbookKind(originalName, mimeType);
+  if (!kind) {
+    throw new BadRequestException('仅支持 .xlsx / .csv 文件');
+  }
+  if (/\.xls$/i.test(originalName) && !/\.xlsx$/i.test(originalName)) {
+    throw new BadRequestException('暂不支持 .xls，请另存为 .xlsx 或 .csv 后重试');
+  }
+  try {
+    const workbook = new ExcelJS.Workbook();
+    if (kind === 'csv') {
+      await workbook.csv.read(Readable.from(Buffer.from(buffer)));
+    } else {
+      await workbook.xlsx.load(buffer.buffer as ArrayBuffer);
+    }
+    const sheets = extractSheets(workbook);
+    if (sheets.length === 0) {
+      throw new BadRequestException('文件中没有可用的工作表数据');
+    }
+    return sheets;
+  } catch (error) {
+    if (error instanceof BadRequestException) throw error;
+    throw new BadRequestException('Excel 文件解析失败，请检查文件格式');
+  }
+}