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/scripts/check-dc-tables.mjs

@@ -203,3 +203,4 @@ checkDCTables();
 
 
 
+

backend/scripts/create-tool-c-ai-history-table.mjs

@@ -155,3 +155,4 @@ createAiHistoryTable()
 
 
 
+

backend/scripts/create-tool-c-table.js

@@ -142,3 +142,4 @@ createToolCTable()
 
 
 
+

backend/scripts/create-tool-c-table.mjs

@@ -139,3 +139,4 @@ createToolCTable()
 
 
 
+