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/conditional.py

@@ -128,6 +128,16 @@ def apply_conditional_column(
         
         print(f'  规则{rule_idx}: 匹配 {matched_count} 行 → 值为 {result_value}')
     
+    # ✨ 优化：将新列移到第一个引用列旁边
+    first_ref_col = rules[0]['conditions'][0]['column']  # 使用第一个规则的第一个条件列作为参考
+    original_col_index = result.columns.get_loc(first_ref_col)
+    cols = list(result.columns)
+    # 移除新列（当前在最后）
+    cols.remove(new_column_name)
+    # 插入到原列旁边
+    cols.insert(original_col_index + 1, new_column_name)
+    result = result[cols]
+    
     # 统计结果分布
     print(f'\n结果分布:')
     value_counts = result[new_column_name].value_counts(dropna=False)