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

extraction_service/operations/recode.py

@@ -54,8 +54,13 @@ def apply_recode(
     # 创建结果数据框（避免修改原数据）
     result = df.copy()
     
-    # 应用映射
-    result[target_column] = result[column].map(mapping)
+    # ✨ 优化：如果是创建新列，插入到原列旁边
+    if create_new_column:
+        original_col_index = result.columns.get_loc(column)
+        result.insert(original_col_index + 1, target_column, result[column].map(mapping))
+    else:
+        # 覆盖原列
+        result[target_column] = result[column].map(mapping)
     
     # 统计结果
     mapped_count = result[target_column].notna().sum()
@@ -77,3 +82,4 @@ def apply_recode(
     
     return result
 
+