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

python-microservice/operations/__init__.py

@@ -14,3 +14,4 @@
 
 __version__ = '1.0.0'
 
+

python-microservice/operations/binning.py

@@ -121,3 +121,4 @@ def apply_binning(
     
     return result
 
+

python-microservice/operations/filter.py

@@ -107,3 +107,4 @@ def apply_filter(
     
     return result
 
+

python-microservice/operations/recode.py

@@ -77,3 +77,4 @@ def apply_recode(
     
     return result
 
+