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/controllers/SessionController.ts

@@ -25,6 +25,10 @@ interface SessionIdParams {
   id: string;
 }
 
+interface GetUniqueValuesQuery {
+  column: string;
+}
+
 // ==================== 控制器 ====================
 
 export class SessionController {
@@ -362,6 +366,69 @@ export class SessionController {
       });
     }
   }
+
+  /**
+   * ✨ 获取列的唯一值（用于数值映射）
+   * 
+   * GET /api/v1/dc/tool-c/sessions/:id/unique-values?column=xxx
+   */
+  async getUniqueValues(
+    request: FastifyRequest<{ Params: SessionIdParams; Querystring: GetUniqueValuesQuery }>,
+    reply: FastifyReply
+  ) {
+    try {
+      const { id } = request.params;
+      const { column } = request.query;
+
+      if (!column) {
+        return reply.code(400).send({
+          success: false,
+          error: '缺少column参数',
+        });
+      }
+
+      logger.info(`[SessionController] 获取唯一值: session=${id}, column=${column}`);
+
+      // 1. 获取完整数据
+      const data = await sessionService.getFullData(id);
+
+      // 2. 提取唯一值（去除空值和首尾空格）
+      const values = data.map((row) => row[column]);
+      const cleanedValues = values.map((val) => {
+        if (val === null || val === undefined || val === '') return null;
+        // 如果是字符串，去除首尾空格
+        return typeof val === 'string' ? val.trim() : val;
+      });
+
+      // 3. 去重
+      const uniqueValues = Array.from(new Set(cleanedValues))
+        .filter((v) => v !== null && v !== '' && v !== '(空白)')
+        .sort(); // 排序，方便查看
+
+      logger.info(`[SessionController] 唯一值数量: ${uniqueValues.length}`);
+
+      // 4. 返回结果
+      return reply.send({
+        success: true,
+        data: {
+          column,
+          uniqueValues,
+          count: uniqueValues.length,
+        },
+      });
+    } catch (error: any) {
+      logger.error(`[SessionController] 获取唯一值失败: ${error.message}`);
+
+      const statusCode = error.message.includes('不存在') || error.message.includes('过期')
+        ? 404
+        : 500;
+
+      return reply.code(statusCode).send({
+        success: false,
+        error: error.message || '获取唯一值失败',
+      });
+    }
+  }
 }
 
 // ==================== 导出单例实例 ====================