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/ai-chat/ai-attachment.service.ts

@@ -16,7 +16,6 @@ import { OfficeCliService } from './office-cli.service';
 import { AiAttachment } from './entities';
 
 const MAX_FILE_BYTES = 10 * 1024 * 1024;
-const MAX_EXTRACTED_CHARS = 48 * 1024;
 const MAX_MODEL_IMAGE_BYTES = 20 * 1024 * 1024;
 const ACCEPTED_MIME_TYPES = new Set([
   'image/jpeg',
@@ -165,7 +164,7 @@ export class AiAttachmentService {
       } else {
         parts.push({
           attachment,
-          text: attachment.extractedText?.slice(0, MAX_EXTRACTED_CHARS) || '',
+          text: attachment.extractedText || '',
         });
       }
     }
@@ -290,7 +289,7 @@ export class AiAttachmentService {
   }
 
   private normalizeExtractedText(value: string): string {
-    return value.split('\u0000').join('').replace(/\r\n/g, '\n').trim().slice(0, MAX_EXTRACTED_CHARS);
+    return value.split('\u0000').join('').replace(/\r\n/g, '\n').trim();
   }
 
   private assertDeclaredType(declared: string, detected: string): void {