fix(dc/tool-c): Fix special character handling and improve UX

Major fixes:
- Fix pivot transformation with special characters in column names
- Fix compute column validation for Chinese punctuation
- Fix recode dialog to fetch unique values from full dataset via new API
- Add column mapping mechanism to handle special characters

Database migration:
- Add column_mapping field to dc_tool_c_sessions table
- Migration file: 20251208_add_column_mapping

UX improvements:
- Darken table grid lines for better visibility
- Reduce column width by 40% with tooltip support
- Insert new columns next to source columns
- Preserve original row order after operations
- Add notice about 50-row preview limit

Modified files:
- Backend: SessionService, SessionController, QuickActionService, routes
- Python: pivot.py, compute.py, recode.py, binning.py, conditional.py
- Frontend: DataGrid, RecodeDialog, index.tsx, ag-grid-custom.css
- Database: schema.prisma, migration SQL

Status: Code complete, database migrated, ready for testing

backend/src/modules/dc/tool-c/services/SessionService.ts

@@ -18,6 +18,12 @@ import * as xlsx from 'xlsx';
 
 // ==================== 类型定义 ====================
 
+interface ColumnMapping {
+  originalName: string;
+  safeName: string;
+  displayName: string;
+}
+
 interface SessionData {
   id: string;
   userId: string;
@@ -26,6 +32,7 @@ interface SessionData {
   totalRows: number;
   totalCols: number;
   columns: string[];
+  columnMapping?: ColumnMapping[]; // ✨ 新增：列名映射
   encoding: string | null;
   fileSize: number;
   createdAt: Date;
@@ -102,8 +109,12 @@ export class SessionService {
       const totalRows = data.length;
       const totalCols = Object.keys(data[0] || {}).length;
       const columns = Object.keys(data[0] || {});
+      
+      // ✨ 生成列名映射（解决特殊字符问题）
+      const columnMapping = this.generateColumnMapping(columns);
 
       logger.info(`[SessionService] 解析完成: ${totalRows}行 x ${totalCols}列`);
+      logger.info(`[SessionService] 列名映射: ${columnMapping.length}个列`);
 
       // 4. 上传到OSS（使用平台storage服务）
       const timestamp = Date.now();
@@ -130,6 +141,7 @@ export class SessionService {
           totalRows,
           totalCols,
           columns: columns, // Prisma会自动转换为JSONB
+          columnMapping: JSON.parse(JSON.stringify(columnMapping)), // ✨ 存储列名映射
           encoding: 'utf-8', // 默认utf-8，后续可扩展检测
           fileSize: fileBuffer.length,
           dataStats: JSON.parse(JSON.stringify(dataStats)), // ✨ 存储统计信息（转换为JSON）
@@ -370,12 +382,15 @@ export class SessionService {
 
       // 4. 更新Session元数据
       const newColumns = Object.keys(processedData[0] || {});
+      const newColumnMapping = this.generateColumnMapping(newColumns); // ✨ 重新生成列名映射
+      
       await prisma.dcToolCSession.update({
         where: { id: sessionId },
         data: {
           totalRows: processedData.length,
           totalCols: newColumns.length,
           columns: newColumns,
+          columnMapping: JSON.parse(JSON.stringify(newColumnMapping)), // ✨ 更新列名映射
           updatedAt: new Date(),
         },
       });
@@ -517,6 +532,29 @@ export class SessionService {
     };
   }
 
+  /**
+   * ✨ 生成安全的列名映射
+   * 
+   * 解决特殊字符问题：表头包含括号、等号等特殊字符会导致Python处理失败
+   * 
+   * @param originalColumns - 原始列名数组
+   * @returns 列名映射数组
+   */
+  private generateColumnMapping(originalColumns: string[]): ColumnMapping[] {
+    return originalColumns.map((originalName, index) => {
+      // 安全列名：col_0, col_1, col_2...
+      const safeName = `col_${index}`;
+      // 显示名称：用于前端展示（保持原始名称）
+      const displayName = originalName;
+      
+      return {
+        originalName,
+        safeName,
+        displayName,
+      };
+    });
+  }
+
   /**
    * 检测列的数据类型
    * 
@@ -571,6 +609,7 @@ export class SessionService {
       totalRows: session.totalRows,
       totalCols: session.totalCols,
       columns: session.columns as string[],
+      columnMapping: session.columnMapping as ColumnMapping[] | undefined, // ✨ 返回列名映射
       encoding: session.encoding,
       fileSize: session.fileSize,
       createdAt: session.createdAt,